diff --git a/src/cmd_context/cmd_context.cpp b/src/cmd_context/cmd_context.cpp
index 95c030687..25cb2a541 100644
--- a/src/cmd_context/cmd_context.cpp
+++ b/src/cmd_context/cmd_context.cpp
@@ -1776,6 +1776,7 @@ void cmd_context::validate_model() {
                     continue;
                 }
                 TRACE("model_validate", model_smt2_pp(tout, *this, *(md.get()), 0););
+                IF_VERBOSE(10, verbose_stream() << "model check failed on: " << mk_pp(a, m()) << "\n";);                
                 invalid_model = true;
             }
         }
diff --git a/src/sat/sat_asymm_branch.cpp b/src/sat/sat_asymm_branch.cpp
index 14969adb8..03d121599 100644
--- a/src/sat/sat_asymm_branch.cpp
+++ b/src/sat/sat_asymm_branch.cpp
@@ -115,6 +115,7 @@ namespace sat {
     }
 
     bool asymm_branch::process(clause & c) {
+        if (c.is_blocked()) return true;
         TRACE("asymm_branch_detail", tout << "processing: " << c << "\n";);
         SASSERT(s.scope_lvl() == 0);
         SASSERT(s.m_qhead == s.m_trail.size());
diff --git a/src/sat/sat_clause.cpp b/src/sat/sat_clause.cpp
index 206dfcf40..43d614e38 100644
--- a/src/sat/sat_clause.cpp
+++ b/src/sat/sat_clause.cpp
@@ -60,15 +60,15 @@ namespace sat {
     }
 
     bool clause::contains(literal l) const {
-        for (unsigned i = 0; i < m_size; i++)
-            if (m_lits[i] == l)
+        for (literal l2 : *this) 
+            if (l2 == l) 
                 return true;
         return false;
     }
 
     bool clause::contains(bool_var v) const {
-        for (unsigned i = 0; i < m_size; i++)
-            if (m_lits[i].var() == v)
+        for (literal l : *this) 
+            if (l.var() == v)
                 return true;
         return false;
     }
@@ -87,8 +87,7 @@ namespace sat {
     }
 
     bool clause::satisfied_by(model const & m) const {
-        for (unsigned i = 0; i < m_size; i++) {
-            literal  l = m_lits[i];
+        for (literal l : *this) {
             if (l.sign()) {
                 if (m[l.var()] == l_false)
                     return true;
@@ -197,14 +196,13 @@ namespace sat {
         if (c.was_removed()) out << "x";
         if (c.strengthened()) out << "+";
         if (c.is_learned()) out << "*";
+        if (c.is_blocked()) out << "b";
         return out;
     }
 
     std::ostream & operator<<(std::ostream & out, clause_vector const & cs) {
-        clause_vector::const_iterator it  = cs.begin();
-        clause_vector::const_iterator end = cs.end();
-        for (; it != end; ++it) {
-            out << *(*it) << "\n";
+        for (clause *cp : cs) {
+            out << *cp << "\n";
         }
         return out;
     }
@@ -226,12 +224,12 @@ namespace sat {
     }
 
     std::ostream & operator<<(std::ostream & out, clause_wrapper const & c) {
-        out << "(";
-        for (unsigned i = 0; i < c.size(); i++) {
-            if (i > 0) out << " ";
-            out << c[i];
+        if (c.is_binary()) {
+            out << "(" << c[0] << " " << c[1] << ")";
+        }
+        else {
+            out << c.get_clause()->id() << ": " << *c.get_clause();
         }
-        out << ")";
         return out;
     }
 
diff --git a/src/sat/sat_clause.h b/src/sat/sat_clause.h
index 8fd7b52cf..dd8028ec2 100644
--- a/src/sat/sat_clause.h
+++ b/src/sat/sat_clause.h
@@ -33,6 +33,10 @@ namespace sat {
 
     class clause_allocator;
 
+    class clause;
+
+    std::ostream & operator<<(std::ostream & out, clause const & c);
+
     class clause {
         friend class clause_allocator;
         friend class tmp_clause;
@@ -61,8 +65,8 @@ namespace sat {
         literal & operator[](unsigned idx) { SASSERT(idx < m_size); return m_lits[idx]; }
         literal const & operator[](unsigned idx) const { SASSERT(idx < m_size); return m_lits[idx]; }
         bool is_learned() const { return m_learned; }
-        void unset_learned() { SASSERT(is_learned()); m_learned = false; }
-        void shrink(unsigned num_lits) { SASSERT(num_lits <= m_size); if (num_lits < m_size) { m_size = num_lits; mark_strengthened(); } }
+        void unset_learned() { if (id() == 642277) std::cout << "unlearn " << *this << "\n"; SASSERT(is_learned()); m_learned = false; }
+        void shrink(unsigned num_lits) { SASSERT(num_lits <= m_size); if (num_lits < m_size) { m_size = num_lits; mark_strengthened(); if (id() == 642277) std::cout << "shrink " << *this << "\n"; } }
         bool strengthened() const { return m_strengthened; }
         void mark_strengthened() { m_strengthened = true; update_approx(); }
         void unmark_strengthened() { m_strengthened = false; }
@@ -101,7 +105,6 @@ namespace sat {
         void set_reinit_stack(bool f) { m_reinit_stack = f; }
     };
 
-    std::ostream & operator<<(std::ostream & out, clause const & c);
     std::ostream & operator<<(std::ostream & out, clause_vector const & cs);
 
     class bin_clause {
@@ -180,6 +183,7 @@ namespace sat {
         bool contains(literal l) const;
         bool contains(bool_var v) const;
         clause * get_clause() const { SASSERT(!is_binary()); return m_cls; }
+        bool was_removed() const { return !is_binary() && get_clause()->was_removed(); }
     };
 
     typedef svector<clause_wrapper> clause_wrapper_vector;
diff --git a/src/sat/sat_cleaner.cpp b/src/sat/sat_cleaner.cpp
index 286cc1661..e52cf139f 100644
--- a/src/sat/sat_cleaner.cpp
+++ b/src/sat/sat_cleaner.cpp
@@ -137,7 +137,8 @@ namespace sat {
                     SASSERT(s.value(c[0]) == l_undef && s.value(c[1]) == l_undef);
                     if (new_sz == 2) {
                         TRACE("cleanup_bug", tout << "clause became binary: " << c[0] << " " << c[1] << "\n";);
-                        s.mk_bin_clause(c[0], c[1], c.is_learned());
+                        if (!c.is_blocked()) 
+                            s.mk_bin_clause(c[0], c[1], c.is_learned());
                         s.del_clause(c);
                     }
                     else {
diff --git a/src/sat/sat_config.cpp b/src/sat/sat_config.cpp
index 08f1b86b2..d0618d265 100644
--- a/src/sat/sat_config.cpp
+++ b/src/sat/sat_config.cpp
@@ -146,9 +146,10 @@ namespace sat {
         m_minimize_lemmas = p.minimize_lemmas();
         m_core_minimize   = p.core_minimize();
         m_core_minimize_partial   = p.core_minimize_partial();
-        m_drat_check      = p.drat_check();
+        m_drat_check_unsat  = p.drat_check_unsat();
+        m_drat_check_sat  = p.drat_check_sat();
         m_drat_file       = p.drat_file();
-        m_drat            = (m_drat_check || m_drat_file != symbol("")) && p.threads() == 1;
+        m_drat            = (m_drat_check_unsat || m_drat_file != symbol("") || m_drat_check_sat) && p.threads() == 1;
         m_dyn_sub_res     = p.dyn_sub_res();
 
         // Parameters used in Liang, Ganesh, Poupart, Czarnecki AAAI 2016.
diff --git a/src/sat/sat_config.h b/src/sat/sat_config.h
index a6db0cddc..17ce7a569 100644
--- a/src/sat/sat_config.h
+++ b/src/sat/sat_config.h
@@ -105,7 +105,8 @@ namespace sat {
         bool               m_core_minimize_partial;
         bool               m_drat;
         symbol             m_drat_file;
-        bool               m_drat_check;
+        bool               m_drat_check_unsat;
+        bool               m_drat_check_sat;
 
         bool               m_dimacs_display;
         bool               m_dimacs_inprocess_display;
diff --git a/src/sat/sat_drat.cpp b/src/sat/sat_drat.cpp
index d3864fd8b..fd2b6fa79 100644
--- a/src/sat/sat_drat.cpp
+++ b/src/sat/sat_drat.cpp
@@ -27,7 +27,10 @@ namespace sat {
     drat::drat(solver& s):
         s(s),
         m_out(0),
-        m_inconsistent(false)
+        m_inconsistent(false),
+        m_check_unsat(false),
+        m_check_sat(false),
+        m_check(false)
     {
         if (s.m_config.m_drat && s.m_config.m_drat_file != symbol()) {
             m_out = alloc(std::ofstream, s.m_config.m_drat_file.str().c_str());
@@ -44,6 +47,12 @@ namespace sat {
         }
     }
 
+    void drat::updt_config() {
+        m_check_unsat = s.m_config.m_drat_check_unsat;
+        m_check_sat = s.m_config.m_drat_check_sat;
+        m_check = m_check_unsat || m_check_sat;
+    }
+
     std::ostream& operator<<(std::ostream& out, drat::status st) {
         switch (st) {
         case drat::status::learned:  return out << "l";
@@ -88,7 +97,7 @@ namespace sat {
     }
 
     void drat::append(literal l, status st) {
-        IF_VERBOSE(10, trace(verbose_stream(), 1, &l, st););
+        IF_VERBOSE(20, trace(verbose_stream(), 1, &l, st););
         if (st == status::learned) {
             verify(1, &l);
         }
@@ -100,7 +109,7 @@ namespace sat {
 
     void drat::append(literal l1, literal l2, status st) {
         literal lits[2] = { l1, l2 };
-        IF_VERBOSE(10, trace(verbose_stream(), 2, lits, st););
+        IF_VERBOSE(20, trace(verbose_stream(), 2, lits, st););
         if (st == status::deleted) {
             // noop
             // don't record binary as deleted.
@@ -131,7 +140,7 @@ namespace sat {
 
     void drat::append(clause& c, status st) {
         unsigned n = c.size();
-        IF_VERBOSE(10, trace(verbose_stream(), n, c.begin(), st););
+        IF_VERBOSE(20, trace(verbose_stream(), n, c.begin(), st););
 
         if (st == status::learned) {
             verify(n, c.begin());
@@ -270,7 +279,7 @@ namespace sat {
     }
 
     void drat::verify(unsigned n, literal const* c) {
-        if (!is_drup(n, c) && !is_drat(n, c)) {
+        if (m_check_unsat && !is_drup(n, c) && !is_drat(n, c)) {
             std::cout << "Verification failed\n";
             UNREACHABLE();
             //display(std::cout);
@@ -412,7 +421,7 @@ namespace sat {
 
     void drat::add() {
         if (m_out) (*m_out) << "0\n";
-        if (s.m_config.m_drat_check) {
+        if (m_check_unsat) {
             SASSERT(m_inconsistent);
         }
     }
@@ -420,7 +429,7 @@ namespace sat {
         declare(l);
         status st = get_status(learned);
         if (m_out) dump(1, &l, st);
-        if (s.m_config.m_drat_check) append(l, st);
+        if (m_check) append(l, st);
     }
     void drat::add(literal l1, literal l2, bool learned) {
         declare(l1);
@@ -428,17 +437,17 @@ namespace sat {
         literal ls[2] = {l1, l2};
         status st = get_status(learned);
         if (m_out) dump(2, ls, st);
-        if (s.m_config.m_drat_check) append(l1, l2, st);
+        if (m_check) append(l1, l2, st);
     }
     void drat::add(clause& c, bool learned) {
         TRACE("sat", tout << "add: " << c << "\n";);
         for (unsigned i = 0; i < c.size(); ++i) declare(c[i]);
         status st = get_status(learned);
         if (m_out) dump(c.size(), c.begin(), st);
-        if (s.m_config.m_drat_check) append(c, get_status(learned));
+        if (m_check_unsat) append(c, get_status(learned));
     }
     void drat::add(literal_vector const& lits, svector<premise> const& premises) {
-        if (s.m_config.m_drat_check) {
+        if (m_check) {
             switch (lits.size()) {
             case 0: add(); break;
             case 1: append(lits[0], status::external); break;
@@ -453,7 +462,7 @@ namespace sat {
     void drat::add(literal_vector const& c) {
         for (unsigned i = 0; i < c.size(); ++i) declare(c[i]);
         if (m_out) dump(c.size(), c.begin(), status::learned);
-        if (s.m_config.m_drat_check) {
+        if (m_check) {
             switch (c.size()) {
             case 0: add(); break;
             case 1: append(c[0], status::learned); break;
@@ -469,20 +478,25 @@ namespace sat {
 
     void drat::del(literal l) {
         if (m_out) dump(1, &l, status::deleted);
-        if (s.m_config.m_drat_check) append(l, status::deleted);
+        if (m_check_unsat) append(l, status::deleted);
     }
     void drat::del(literal l1, literal l2) {
         literal ls[2] = {l1, l2};
         if (m_out) dump(2, ls, status::deleted);
-        if (s.m_config.m_drat_check) 
+        if (m_check) 
             append(l1, l2, status::deleted);
     }
     void drat::del(clause& c) {
         TRACE("sat", tout << "del: " << c << "\n";);
         if (m_out) dump(c.size(), c.begin(), status::deleted);
-        if (s.m_config.m_drat_check) {
+        if (m_check) {
             clause* c1 = s.m_cls_allocator.mk_clause(c.size(), c.begin(), c.is_learned()); 
             append(*c1, status::deleted);
         }
     }
+    
+    void drat::check_model(model const& m) {
+        std::cout << "check model on " << m_proof.size() << "\n";
+    }
+
 }
diff --git a/src/sat/sat_drat.h b/src/sat/sat_drat.h
index 2765d104c..64d796839 100644
--- a/src/sat/sat_drat.h
+++ b/src/sat/sat_drat.h
@@ -52,6 +52,7 @@ namespace sat {
         vector<watch>           m_watches;
         svector<lbool>          m_assignment;
         bool                    m_inconsistent;
+        bool                    m_check_unsat, m_check_sat, m_check;
 
         void dump(unsigned n, literal const* c, status st);
         void append(literal l, status st);
@@ -78,6 +79,8 @@ namespace sat {
     public:
         drat(solver& s);
         ~drat();  
+
+        void updt_config();
         void add();
         void add(literal l, bool learned);
         void add(literal l1, literal l2, bool learned);
@@ -89,6 +92,8 @@ namespace sat {
         void del(literal l);
         void del(literal l1, literal l2);
         void del(clause& c);
+
+        void check_model(model const& m);
     };
 
 };
diff --git a/src/sat/sat_lookahead.h b/src/sat/sat_lookahead.h
index 56d2e94a1..baf3ed660 100644
--- a/src/sat/sat_lookahead.h
+++ b/src/sat/sat_lookahead.h
@@ -246,8 +246,8 @@ namespace sat {
         inline bool is_true(literal l) const { return is_true_at(l, m_level); }
         inline void set_true(literal l) { m_stamp[l.var()] = m_level + l.sign(); }
         inline void set_undef(literal l) { m_stamp[l.var()] = 0; }
-        inline unsigned get_level(literal l) const { return m_stamp[l.var()] & UINT_MAX - 1; }
-        void set_level(literal d, literal s) { m_stamp[d.var()] = (m_stamp[s.var()] & ~0x1) + d.sign(); }
+        inline unsigned get_level(literal l) const { return m_stamp[l.var()] & ~0x1; }
+        void set_level(literal d, literal s) { m_stamp[d.var()] = get_level(s) + d.sign(); }
         lbool value(literal l) const { return is_undef(l) ? l_undef : is_true(l) ? l_true : l_false; }
         
         // set the level within a scope of the search.
diff --git a/src/sat/sat_model_converter.cpp b/src/sat/sat_model_converter.cpp
index eb60e209a..2cc9ceffc 100644
--- a/src/sat/sat_model_converter.cpp
+++ b/src/sat/sat_model_converter.cpp
@@ -18,11 +18,12 @@ Revision History:
 --*/
 #include "sat/sat_model_converter.h"
 #include "sat/sat_clause.h"
+#include "sat/sat_solver.h"
 #include "util/trace.h"
 
 namespace sat {
 
-    model_converter::model_converter() {
+    model_converter::model_converter(): m_solver(nullptr) {
     }
 
     model_converter::~model_converter() {
@@ -50,7 +51,6 @@ namespace sat {
             }
             if (!sat) {
                 m[lit.var()] = lit.sign() ? l_false : l_true;
-                // if (lit.var() == 258007) std::cout << "flip " << lit << " " << m[lit.var()] << " @ " << i << " of " << c << " from overall size " << sz << "\n";
             }
         }
     }
@@ -58,6 +58,8 @@ namespace sat {
     void model_converter::operator()(model & m) const {
         vector<entry>::const_iterator begin = m_entries.begin();
         vector<entry>::const_iterator it    = m_entries.end();
+        bool first = true;
+        VERIFY(!m_solver || m_solver->check_clauses(m));
         while (it != begin) {
             --it;
             SASSERT(it->get_kind() != ELIM_VAR || m[it->var()] == l_undef);
@@ -70,14 +72,18 @@ namespace sat {
             for (literal l : it->m_clauses) {
                 if (l == null_literal) {
                     // end of clause
-                    elim_stack* s = it->m_elim_stack[index];
+                    elim_stack* st = it->m_elim_stack[index];
                     if (!sat) {                        
                         m[it->var()] = var_sign ? l_false : l_true;
                     }
-                    if (s) {
-                        process_stack(m, clause, s->stack());
+                    if (st) {
+                        process_stack(m, clause, st->stack());
                     }
                     sat = false;
+                    if (first && m_solver && !m_solver->check_clauses(m)) {
+                        display(std::cout, *it) << "\n";
+                        first = false;
+                    }
                     ++index;
                     clause.reset();
                     continue;                    
@@ -95,8 +101,12 @@ namespace sat {
                 else if (!sat && v != it->var() && m[v] == l_undef) {
                     // clause can be satisfied by assigning v.
                     m[v] = sign ? l_false : l_true;
-                    // if (v == 258007) std::cout << "set undef " << v << " to " << m[v] << " in " << clause << "\n";
+                    // if (first) std::cout << "set: " << l << "\n";
                     sat = true;
+                    if (first && m_solver && !m_solver->check_clauses(m)) {
+                        display(std::cout, *it)  << "\n";;
+                        first = false;
+                    }
                 }
             }
             DEBUG_CODE({
@@ -217,10 +227,12 @@ namespace sat {
                 it2++;
                 for (; it2 != end; ++it2) {
                     SASSERT(it2->var() != it->var());
+                    if (it2->var() == it->var()) return false;
                     for (literal l : it2->m_clauses) {
                         CTRACE("sat_model_converter", l.var() == it->var(), tout << "var: " << it->var() << "\n"; display(tout););
                         SASSERT(l.var() != it->var());
                         SASSERT(l == null_literal || l.var() < num_vars);
+                        if (it2->var() == it->var()) return false;
                     }
                 }
             }
@@ -229,31 +241,105 @@ namespace sat {
     }
 
     void model_converter::display(std::ostream & out) const {
-        out << "(sat::model-converter";
+        out << "(sat::model-converter\n";
+        bool first = true;
         for (auto & entry : m_entries) {
-            out << "\n  (" << (entry.get_kind() == ELIM_VAR ? "elim" : "blocked") << " " << entry.var();
-            bool start = true;
-            for (literal l : entry.m_clauses) {
-                if (start) {
-                    out << "\n    (";
-                    start = false;
-                }
-                else {
-                    if (l != null_literal)
-                        out << " ";
-                }
-                if (l == null_literal) {
-                    out << ")";
-                    start = true;
-                    continue;
-                }
-                out << l;
-            }
-            out << ")";
+            if (first) first = false; else out << "\n";
+            display(out, entry);
         }
         out << ")\n";
     }
 
+    std::ostream& model_converter::display(std::ostream& out, entry const& entry) const {
+        out << "  (";
+        switch (entry.get_kind()) {
+        case ELIM_VAR: out << "elim"; break;
+        case BLOCK_LIT: out << "blocked"; break;
+        case CCE: out << "cce"; break;
+        case ACCE: out << "acce"; break;
+        }
+        out << " " << entry.var();
+        bool start = true;
+        unsigned index = 0;
+        for (literal l : entry.m_clauses) {
+            if (start) {
+                out << "\n    (";
+                start = false;
+            }
+            else {
+                if (l != null_literal)
+                    out << " ";
+            }
+            if (l == null_literal) {
+                out << ")";
+                start = true;
+                elim_stack* st = entry.m_elim_stack[index];
+                if (st) {
+                    elim_stackv const& stack = st->stack();
+                    unsigned sz = stack.size();
+                    for (unsigned i = sz; i-- > 0; ) {
+                        out << "\n   " << stack[i].first << " " << stack[i].second;
+                    }
+                }
+                ++index;
+                continue;
+            }
+            out << l;
+        }
+        out << ")";
+        for (literal l : entry.m_clauses) {
+            if (l != null_literal) {
+                if (m_solver && m_solver->was_eliminated(l.var())) out << "\neliminated: " << l;
+            }
+        }
+        return out;
+    }
+
+    void model_converter::validate_is_blocked(entry const& e, clause const& c) {
+        if (c.is_blocked() || c.is_learned()) return;
+        unsigned index = 0;
+        literal lit = null_literal;
+        bool_var v = e.var();
+        for (literal l : c) {
+            if (l.var() == v) {
+                lit = l;
+                break;
+            }
+        }
+        if (lit == null_literal) return;
+
+        bool sat = false;
+        for (literal l : e.m_clauses) {
+            if (l == null_literal) {
+                if (!sat) {
+                    display(std::cout << "clause is not blocked\n", e) << "\n";
+                    std::cout << c << "\n";
+                }
+                sat = false;
+                elim_stack* st = e.m_elim_stack[index];
+                if (st) {
+                    elim_stackv const& stack = st->stack();
+                    unsigned sz = stack.size();
+                    for (unsigned i = sz; i-- > 0; ) {
+                        // verify ...
+                    }
+
+                }
+                ++index;
+                continue;
+            }
+            if (sat) {
+                continue;
+            }
+            if (l.var() == v) {
+                sat = l == lit;
+            }
+            else {
+                sat = c.contains(~l);
+            }
+        }
+    }
+
     void model_converter::copy(model_converter const & src) {
         reset();
         m_entries.append(src.m_entries);
diff --git a/src/sat/sat_model_converter.h b/src/sat/sat_model_converter.h
index dcbe450a8..cdd8c60ed 100644
--- a/src/sat/sat_model_converter.h
+++ b/src/sat/sat_model_converter.h
@@ -36,6 +36,9 @@ namespace sat {
        This is a low-level model_converter. Given a mapping from bool_var to expr,
        it can be converted into a general Z3 model_converter
     */
+
+    class solver;
+
     class model_converter {
         
     public:
@@ -55,11 +58,11 @@ namespace sat {
             elim_stackv const& stack() const { return m_stack; }
         };
 
-        enum kind { ELIM_VAR = 0, BLOCK_LIT };
+        enum kind { ELIM_VAR = 0, BLOCK_LIT, CCE, ACCE };
         class entry {
             friend class model_converter;
-            unsigned           m_var:31;
-            unsigned           m_kind:1;
+            unsigned           m_var:30;
+            unsigned           m_kind:2;
             literal_vector     m_clauses; // the different clauses are separated by null_literal
             sref_vector<elim_stack>  m_elim_stack;
             entry(kind k, bool_var v):m_var(v), m_kind(k) {}
@@ -75,12 +78,18 @@ namespace sat {
         };
     private:
         vector<entry>          m_entries;
+        solver const*          m_solver;
 
         void process_stack(model & m, literal_vector const& clause, elim_stackv const& stack) const;
 
+        std::ostream& display(std::ostream & out, entry const& entry) const;
+
+        void validate_is_blocked(entry const& e, clause const& c);
+
     public:
         model_converter();
         ~model_converter();
+        void set_solver(solver const* s) { m_solver = s; }
         void operator()(model & m) const;
         model_converter& operator=(model_converter const& other);
 
diff --git a/src/sat/sat_params.pyg b/src/sat/sat_params.pyg
index d803617f5..6be3b35ba 100644
--- a/src/sat/sat_params.pyg
+++ b/src/sat/sat_params.pyg
@@ -28,7 +28,8 @@ def_module_params('sat',
                           ('threads', UINT, 1, 'number of parallel threads to use'),
                           ('dimacs.core', BOOL, False, 'extract core from DIMACS benchmarks'),
                           ('drat.file', SYMBOL, '', 'file to dump DRAT proofs'),
-                          ('drat.check', BOOL, False, 'build up internal proof and check'),
+                          ('drat.check_unsat', BOOL, False, 'build up internal proof and check'),
+                          ('drat.check_sat', BOOL, False, 'build up internal trace, check satisfying model'),
                           ('cardinality.solver', BOOL, False, 'use cardinality solver'),
                           ('pb.solver', SYMBOL, 'circuit', 'method for handling Pseudo-Boolean constraints: circuit (arithmetical circuit), sorting (sorting circuit), totalizer (use totalizer encoding), solver (use SMT solver)'),
                           ('xor.solver', BOOL, False, 'use xor solver'),
diff --git a/src/sat/sat_scc.cpp b/src/sat/sat_scc.cpp
index 544ef1f66..089c5c55f 100644
--- a/src/sat/sat_scc.cpp
+++ b/src/sat/sat_scc.cpp
@@ -107,7 +107,7 @@ namespace sat {
                 frame & fr = frames.back();
                 unsigned l_idx = fr.m_lidx;
                 if (!fr.m_first) {
-                    SASSERT(fr.m_it->is_binary_clause());
+                    SASSERT(fr.m_it->is_binary_unblocked_clause());
                     // after visiting child
                     literal l2 = fr.m_it->get_literal();
                     unsigned l2_idx = l2.index();
@@ -118,7 +118,7 @@ namespace sat {
                 }
                 fr.m_first = false;
                 while (fr.m_it != fr.m_end) {
-                    if (!fr.m_it->is_binary_clause()) {
+                    if (!fr.m_it->is_binary_unblocked_clause()) {
                         fr.m_it++;
                         continue;
                     }
diff --git a/src/sat/sat_simplifier.cpp b/src/sat/sat_simplifier.cpp
index 09cb063f5..57ecc5bac 100644
--- a/src/sat/sat_simplifier.cpp
+++ b/src/sat/sat_simplifier.cpp
@@ -201,18 +201,19 @@ namespace sat {
         CASSERT("sat_solver", s.check_invariant());
         m_need_cleanup = false;
         m_use_list.init(s.num_vars());
-        m_learned_in_use_lists = false;
+        m_learned_in_use_lists = learned;
         if (learned) {
             register_clauses(s.m_learned);
-            m_learned_in_use_lists = true;
         }
         register_clauses(s.m_clauses);
 
-        if (!learned && (bce_enabled() || bca_enabled()))
+        if (bce_enabled() || abce_enabled() || bca_enabled()) {
             elim_blocked_clauses();
+        }
 
-        if (!learned)
+        if (!learned) {
             m_num_calls++;
+        }
 
         m_sub_counter  = m_subsumption_limit;
         m_elim_counter = m_res_limit;
@@ -457,7 +458,7 @@ namespace sat {
         literal_vector::iterator l_it = m_bs_ls.begin();
         for (; it != end; ++it, ++l_it) {
             clause & c2 = *(*it);
-            if (!c2.was_removed() && *l_it == null_literal) {
+            if (!c2.was_removed() && !c1.is_blocked() && *l_it == null_literal) {
                 // c2 was subsumed
                 if (c1.is_learned() && !c2.is_learned())
                     c1.unset_learned();
@@ -713,7 +714,7 @@ namespace sat {
             // should not traverse wlist using iterators, since back_subsumption1 may add new binary clauses there
             for (unsigned j = 0; j < wlist.size(); j++) {
                 watched w  = wlist[j];
-                if (w.is_binary_clause()) {
+                if (w.is_binary_unblocked_clause()) {
                     literal l2 = w.get_literal();
                     if (l.index() < l2.index()) {
                         m_dummy.set(l, l2, w.is_learned());
@@ -959,8 +960,10 @@ namespace sat {
         void operator()() {
             if (s.bce_enabled())
                 block_clauses();
+            if (s.abce_enabled())
+                cce<false>();
             if (s.cce_enabled()) 
-                cce();
+                cce<true>();
             if (s.bca_enabled())
                 bca();
     }
@@ -1053,21 +1056,22 @@ namespace sat {
             for (watched & w : s.get_wlist(l)) {
                 // when adding a blocked clause, then all non-learned clauses have to be considered for the
                 // resolution intersection.
-                bool process_bin = adding ? (w.is_binary_clause() && !w.is_learned()) : w.is_binary_unblocked_clause();
+                bool process_bin = adding ? w.is_binary_clause() : w.is_binary_unblocked_clause();
                 if (process_bin) {
                     literal lit = w.get_literal();
-                    if (s.is_marked(~lit) && lit != ~l) continue;
+                    if (s.is_marked(~lit) && lit != ~l) {
+                        continue; // tautology
+                    }
                     if (!first || s.is_marked(lit)) {
                         inter.reset();
-                        return false;
+                        return false; // intersection is empty or does not add anything new.
                     }
                     first = false;
                     inter.push_back(lit);
                 }
             }
             clause_use_list & neg_occs = s.m_use_list.get(~l);
-            clause_use_list::iterator it = neg_occs.mk_iterator();
-            for (; !it.at_end(); it.next()) {
+            for (auto it = neg_occs.mk_iterator(); !it.at_end(); it.next()) {
                 bool tautology = false;
                 clause & c = it.curr();
                 if (c.is_blocked() && !adding) continue;
@@ -1098,6 +1102,30 @@ namespace sat {
             return first;
         }
 
+        bool check_abce_tautology(literal l) {
+            if (!process_var(l.var())) return false;
+            for (watched & w : s.get_wlist(l)) {
+                if (w.is_binary_unblocked_clause()) {
+                    literal lit = w.get_literal();
+                    if (!s.is_marked(~lit) || lit == ~l) return false;
+                }
+            }
+            clause_use_list & neg_occs = s.m_use_list.get(~l);
+            for (auto it = neg_occs.mk_iterator(); !it.at_end(); it.next()) {
+                clause & c = it.curr();
+                if (c.is_blocked()) continue;
+                bool tautology = false;
+                for (literal lit : c) {
+                    if (s.is_marked(~lit) && lit != ~l) {
+                        tautology = true;
+                        break;
+                    }
+                }
+                if (!tautology) return false;
+            }
+            return true;
+        }
+
         /*
          * C \/ l     l \/ lit
          * -------------------
@@ -1146,59 +1174,101 @@ namespace sat {
             return false;
         }
 
-        bool cla(literal& blocked) {
+        bool above_threshold(unsigned sz0) const {
+            return sz0 * 10 < m_covered_clause.size();
+        }
+
+        template<bool use_ri>
+        bool cla(literal& blocked, model_converter::kind& k) {
             bool is_tautology = false;
             for (literal l : m_covered_clause) s.mark_visited(l);
             unsigned num_iterations = 0, sz;
             m_elim_stack.reset();
             m_ala_qhead = 0;
+            k = model_converter::CCE;
+            unsigned sz0 = m_covered_clause.size();
+
+            /*
+             * For blocked clause elimination with asymmetric literal addition (ABCE) 
+             * it suffices to check if one of the original
+             * literals in the clause is blocked modulo the additional literals added to the clause.
+             * So we record sz0, the original set of literals in the clause, mark additional literals, 
+             * and then check if any of the first sz0 literals are blocked.
+             */
+            if (s.abce_enabled() && !use_ri) {
+                add_ala();               
+                for (unsigned i = 0; i < sz0; ++i) {
+                    if (check_abce_tautology(m_covered_clause[i])) {
+                        blocked = m_covered_clause[i];
+                        is_tautology = true;
+                        break;
+                    }
+                }
+                k = model_converter::BLOCK_LIT; // actually ABCE
+                for (literal l : m_covered_clause) s.unmark_visited(l);
+                m_covered_clause.shrink(sz0);
+                return is_tautology;
+            }
+
+            /*
+             * For CCE we add the resolution intersection while checking if the clause becomes a tautology.
+             * For ACCE, in addition, we add literals.
+             */
             do {
                 do {
+                    if (above_threshold(sz0)) break;
                     ++num_iterations;
                     sz = m_covered_clause.size();
                     is_tautology = add_cla(blocked);
                 }
                 while (m_covered_clause.size() > sz && !is_tautology);
+                if (above_threshold(sz0)) break;
                 if (s.acce_enabled() && !is_tautology) {
                     sz = m_covered_clause.size();
                     add_ala();
+                    k = model_converter::ACCE;
                 }
             }
             while (m_covered_clause.size() > sz && !is_tautology);
+            // if (is_tautology) std::cout << num_iterations << " " << m_covered_clause.size() << " " << sz0 << " " << is_tautology << " " << m_elim_stack.size() << "\n";
             for (literal l : m_covered_clause) s.unmark_visited(l);
-            // if (is_tautology) std::cout << "taut: " << num_iterations << " " << m_covered_clause.size() << " " << m_elim_stack.size() << "\n";
-            return is_tautology;
+            return is_tautology && !above_threshold(sz0);
         }
 
         // perform covered clause elimination.
         // first extract the covered literal addition (CLA).
         // then check whether the CLA is blocked.
-        bool cce(clause& c, literal& blocked) {
+        template<bool use_ri>
+        bool cce(clause& c, literal& blocked, model_converter::kind& k) {
             m_covered_clause.reset();
             for (literal l : c) m_covered_clause.push_back(l);
-            return cla(blocked);
+            return cla<use_ri>(blocked, k);
         }
 
-        bool cce(literal l1, literal l2, literal& blocked) {
+        template<bool use_ri>
+        bool cce(literal l1, literal l2, literal& blocked, model_converter::kind& k) {
             m_covered_clause.reset();
             m_covered_clause.push_back(l1);
             m_covered_clause.push_back(l2);
-            return cla(blocked);            
+            return cla<use_ri>(blocked, k);            
         }
         
+        template<bool use_ri>
         void cce() {
             insert_queue();
-            cce_clauses();
-            cce_binary();
+            cce_clauses<use_ri>();
+            cce_binary<use_ri>();
         }
 
+        template<bool use_ri>
         void cce_binary() {
             while (!m_queue.empty() && m_counter >= 0) {
                 s.checkpoint();
-                process_cce_binary(m_queue.next());
+                process_cce_binary<use_ri>(m_queue.next());
             }
         }
 
+        template<bool use_ri>
         void process_cce_binary(literal l) {
             literal blocked = null_literal;
             watch_list & wlist = s.get_wlist(~l);
@@ -1206,15 +1276,15 @@ namespace sat {
             watch_list::iterator it  = wlist.begin();
             watch_list::iterator it2 = it;
             watch_list::iterator end = wlist.end();
-
+            model_converter::kind k;
             for (; it != end; ++it) {
                 if (!it->is_binary_clause() || it->is_blocked()) {
                     INC();
                     continue;
                 }
                 literal l2 = it->get_literal();
-                if (cce(l, l2, blocked)) {
-                    block_covered_binary(it, l, blocked);
+                if (cce<use_ri>(l, l2, blocked, k)) {
+                    block_covered_binary(it, l, blocked, k);
                     s.m_num_covered_clauses++;
                 }
                 else {
@@ -1225,13 +1295,15 @@ namespace sat {
         }
 
 
+        template<bool use_ri>
         void cce_clauses() {
             m_to_remove.reset();
             literal blocked;
+            model_converter::kind k;
             for (clause* cp : s.s.m_clauses) {
                 clause& c = *cp;
-                if (!c.was_removed() && !c.is_blocked() && cce(c, blocked)) {
-                    block_covered_clause(c, blocked);
+                if (!c.was_removed() && !c.is_blocked() && cce<use_ri>(c, blocked, k)) {
+                    block_covered_clause(c, blocked, k);
                     s.m_num_covered_clauses++;                    
                 }
             }
@@ -1242,10 +1314,10 @@ namespace sat {
         }
 
 
-        void prepare_block_clause(clause& c, literal l, model_converter::entry*& new_entry) {
+        void prepare_block_clause(clause& c, literal l, model_converter::entry*& new_entry, model_converter::kind k) {
             TRACE("blocked_clause", tout << "new blocked clause: " << c << "\n";);
             if (new_entry == 0)
-                new_entry = &(mc.mk(model_converter::BLOCK_LIT, l.var()));
+                new_entry = &(mc.mk(k, l.var()));
             m_to_remove.push_back(&c);
             for (literal lit : c) {
                 if (lit != l && process_var(lit.var())) {
@@ -1255,19 +1327,19 @@ namespace sat {
         }
 
         void block_clause(clause& c, literal l, model_converter::entry *& new_entry) {
-            prepare_block_clause(c, l, new_entry);
+            prepare_block_clause(c, l, new_entry, model_converter::BLOCK_LIT);
             mc.insert(*new_entry, c);
         }
 
-        void block_covered_clause(clause& c, literal l) {
+        void block_covered_clause(clause& c, literal l, model_converter::kind k) {
             model_converter::entry * new_entry = 0;
-            prepare_block_clause(c, l, new_entry);
+            prepare_block_clause(c, l, new_entry, k);
             mc.insert(*new_entry, m_covered_clause, m_elim_stack);
         }
         
-        void prepare_block_binary(watch_list::iterator it, literal l1, literal blocked, model_converter::entry*& new_entry) {
+        void prepare_block_binary(watch_list::iterator it, literal l1, literal blocked, model_converter::entry*& new_entry, model_converter::kind k) {
             if (new_entry == 0) 
-                new_entry = &(mc.mk(model_converter::BLOCK_LIT, blocked.var()));
+                new_entry = &(mc.mk(k, blocked.var()));
             literal l2 = it->get_literal();
             TRACE("blocked_clause", tout << "new blocked clause: " << l2 << " " << l1 << "\n";);
             if (s.m_retain_blocked_clauses && !it->is_learned()) {
@@ -1281,13 +1353,13 @@ namespace sat {
         }
         
         void block_binary(watch_list::iterator it, literal l, model_converter::entry *& new_entry) {
-            prepare_block_binary(it, l, l, new_entry);
+            prepare_block_binary(it, l, l, new_entry, model_converter::BLOCK_LIT);
             mc.insert(*new_entry, l, it->get_literal());
         }
 
-        void block_covered_binary(watch_list::iterator it, literal l, literal blocked) {
+        void block_covered_binary(watch_list::iterator it, literal l, literal blocked, model_converter::kind k) {
             model_converter::entry * new_entry = 0;
-            prepare_block_binary(it, l, blocked, new_entry);
+            prepare_block_binary(it, l, blocked, new_entry, k);
             mc.insert(*new_entry, m_covered_clause, m_elim_stack);
         }
 
@@ -1375,10 +1447,12 @@ namespace sat {
         stopwatch    m_watch;
         unsigned     m_num_blocked_clauses;
         unsigned     m_num_covered_clauses;
+        unsigned     m_num_added_clauses;
         blocked_cls_report(simplifier & s):
             m_simplifier(s),
             m_num_blocked_clauses(s.m_num_blocked_clauses),
-            m_num_covered_clauses(s.m_num_covered_clauses) {
+            m_num_covered_clauses(s.m_num_covered_clauses),
+            m_num_added_clauses(s.m_num_bca) {
             m_watch.start();
         }
 
@@ -1389,6 +1463,8 @@ namespace sat {
                        << (m_simplifier.m_num_blocked_clauses - m_num_blocked_clauses)
                        << " :elim-covered-clauses " 
                        << (m_simplifier.m_num_covered_clauses - m_num_covered_clauses)
+                       << " :added-clauses "
+                       << (m_simplifier.m_num_bca - m_num_added_clauses)
                        << mem_stat()
                        << " :time " << std::fixed << std::setprecision(2) << m_watch.get_seconds() << ")\n";);
         }
@@ -1456,8 +1532,7 @@ namespace sat {
     */
     void simplifier::collect_clauses(literal l, clause_wrapper_vector & r, bool include_blocked) {
         clause_use_list const & cs = m_use_list.get(l);
-        clause_use_list::iterator it = cs.mk_iterator();
-        for (; !it.at_end(); it.next()) {
+        for (auto it = cs.mk_iterator(); !it.at_end(); it.next()) {
             if (!it.curr().is_blocked() || include_blocked) {
                 r.push_back(clause_wrapper(it.curr()));
                 SASSERT(r.back().size() == it.curr().size());
@@ -1703,6 +1778,7 @@ namespace sat {
                     else
                         s.m_stats.m_mk_clause++;
                     clause * new_c = s.m_cls_allocator.mk_clause(m_new_cls.size(), m_new_cls.c_ptr(), false);
+
                     if (s.m_config.m_drat) s.m_drat.add(*new_c, true);
                     s.m_clauses.push_back(new_c);
                     m_use_list.insert(*new_c);
@@ -1769,6 +1845,7 @@ namespace sat {
         m_cce                     = p.cce();
         m_acce                    = p.acce();
         m_bca                     = p.bca();
+        m_bce_delay               = p.bce_delay();
         m_elim_blocked_clauses    = p.elim_blocked_clauses();
         m_elim_blocked_clauses_at = p.elim_blocked_clauses_at();
         m_retain_blocked_clauses  = p.retain_blocked_clauses();
diff --git a/src/sat/sat_simplifier.h b/src/sat/sat_simplifier.h
index c75e45d79..6e120796f 100644
--- a/src/sat/sat_simplifier.h
+++ b/src/sat/sat_simplifier.h
@@ -71,9 +71,11 @@ namespace sat {
         int                    m_elim_counter;
 
         // config
+        bool                   m_abce; // block clauses using asymmetric added literals
         bool                   m_cce;  // covered clause elimination
         bool                   m_acce; // cce with asymetric literal addition
         bool                   m_bca;  // blocked (binary) clause addition. 
+        unsigned               m_bce_delay;
         bool                   m_elim_blocked_clauses;
         unsigned               m_elim_blocked_clauses_at;
         bool                   m_retain_blocked_clauses;
@@ -169,10 +171,11 @@ namespace sat {
         struct blocked_clause_elim;
         void elim_blocked_clauses();
 
-        bool bce_enabled() const { return m_elim_blocked_clauses || m_elim_blocked_clauses_at == m_num_calls || cce_enabled(); }
-        bool acce_enabled() const { return m_acce; }
-        bool cce_enabled() const { return m_cce || acce_enabled(); }
-        bool bca_enabled() const { return m_bca; }
+        bool bce_enabled() const { return !m_learned_in_use_lists && m_num_calls >= m_bce_delay && (m_elim_blocked_clauses || m_elim_blocked_clauses_at == m_num_calls || cce_enabled()); }
+        bool acce_enabled() const { return !m_learned_in_use_lists && m_num_calls >= m_bce_delay && m_acce; }
+        bool cce_enabled() const { return !m_learned_in_use_lists && m_num_calls >= m_bce_delay && (m_cce || acce_enabled()); }
+        bool abce_enabled() const { return !m_learned_in_use_lists && m_num_calls >= m_bce_delay && m_abce; }
+        bool bca_enabled() const { return m_bca && m_learned_in_use_lists; }
         bool elim_vars_bdd_enabled() const { return m_elim_vars_bdd && m_num_calls >= m_elim_vars_bdd_delay; }
         bool elim_vars_enabled() const { return m_elim_vars; }
 
diff --git a/src/sat/sat_simplifier_params.pyg b/src/sat/sat_simplifier_params.pyg
index 6528e6699..4cbd80788 100644
--- a/src/sat/sat_simplifier_params.pyg
+++ b/src/sat/sat_simplifier_params.pyg
@@ -2,10 +2,12 @@ def_module_params(module_name='sat',
                   class_name='sat_simplifier_params',
                   export=True,
                   params=(('elim_blocked_clauses', BOOL, False, 'eliminate blocked clauses'),
+                          ('abce', BOOL, BOOL, False, 'eliminate blocked clauses using asymmmetric literals'),
                           ('cce', BOOL, False, 'eliminate covered clauses'),
                           ('acce', BOOL, False, 'eliminate covered clauses using asymmetric added literals'),
                           ('elim_blocked_clauses_at', UINT, 2, 'eliminate blocked clauses only once at the given simplification round'),
                           ('bca', BOOL, False, 'blocked clause addition - add blocked binary clauses'),
+	                  ('bce_delay', UINT, 0, 'delay eliminate blocked clauses until simplification round'),
                           ('retain_blocked_clauses', BOOL, False, 'retain blocked clauses for propagation, hide them from variable elimination'),
                           ('blocked_clause_limit', UINT, 100000000, 'maximum number of literals visited during blocked clause elimination'),
                           ('resolution.limit', UINT, 500000000, 'approx. maximum number of literals visited during variable elimination'),
diff --git a/src/sat/sat_solver.cpp b/src/sat/sat_solver.cpp
index 1f0015476..0fb4dce93 100644
--- a/src/sat/sat_solver.cpp
+++ b/src/sat/sat_solver.cpp
@@ -23,14 +23,11 @@ Revision History:
 #include "util/luby.h"
 #include "util/trace.h"
 #include "util/max_cliques.h"
+#include "util/gparams.h"
 
 // define to update glue during propagation
 #define UPDATE_GLUE
 
-// define to create a copy of the solver before starting the search
-// useful for checking models
-// #define CLONE_BEFORE_SOLVING
-
 namespace sat {
 
     solver::solver(params_ref const & p, reslimit& l):
@@ -734,15 +731,13 @@ namespace sat {
                 case watched::CLAUSE: {
                     if (value(it->get_blocked_literal()) == l_true) {
                         TRACE("propagate_clause_bug", tout << "blocked literal " << it->get_blocked_literal() << "\n";
-                              clause_offset cls_off = it->get_clause_offset();
-                              clause & c = *(m_cls_allocator.get_clause(cls_off));
-                              tout << c << "\n";);
+                              tout << get_clause(it) << "\n";);
                         *it2 = *it;
                         it2++;
                         break;
                     }
                     clause_offset cls_off = it->get_clause_offset();
-                    clause & c = *(m_cls_allocator.get_clause(cls_off));
+                    clause & c = get_clause(cls_off);
                     TRACE("propagate_clause_bug", tout << "processing... " << c << "\nwas_removed: " << c.was_removed() << "\n";);
                     if (c[0] == not_l)
                         std::swap(c[0], c[1]);
@@ -886,12 +881,10 @@ namespace sat {
             return check_par(num_lits, lits);
         }
         flet<bool> _searching(m_searching, true);
-#ifdef CLONE_BEFORE_SOLVING
-        if (m_mc.empty()) {
-            m_clone = alloc(solver, m_params);
-            SASSERT(m_clone);
+        if (m_mc.empty() && gparams::get().get_bool("model_validate", false)) {
+            m_clone = alloc(solver, m_params, m_rlimit);
+            m_clone->copy(*this);
         }
-#endif
         try {
             init_search();
             if (inconsistent()) return l_false;
@@ -1548,30 +1541,40 @@ namespace sat {
                 m_model[v] = value(v);
         }
         TRACE("sat_mc_bug", m_mc.display(tout););
-        m_mc(m_model);
-        TRACE("sat", for (bool_var v = 0; v < num; v++) tout << v << ": " << m_model[v] << "\n";);
 
-// #ifndef _EXTERNAL_RELEASE
         IF_VERBOSE(10, verbose_stream() << "\"checking model\"\n";);
-        if (!check_model(m_model)) {
+        if (!check_clauses(m_model)) {
             UNREACHABLE();
             throw solver_exception("check model failed");
         }
+        
+        if (m_config.m_drat) m_drat.check_model(m_model);
+
+        // m_mc.set_solver(this);
+        m_mc(m_model);
+
+        
+        if (!check_clauses(m_model)) {
+            std::cout << "failure checking clauses on transformed model\n";
+            UNREACHABLE();
+            throw solver_exception("check model failed");
+        }
+
+        TRACE("sat", for (bool_var v = 0; v < num; v++) tout << v << ": " << m_model[v] << "\n";);
 
         if (m_clone) {
             IF_VERBOSE(SAT_VB_LVL, verbose_stream() << "\"checking model (on original set of clauses)\"\n";);
             if (!m_clone->check_model(m_model))
                 throw solver_exception("check model failed (for cloned solver)");
         }
-// #endif
     }
 
-    bool solver::check_model(model const & m) const {
+    bool solver::check_clauses(model const& m) const {
         bool ok = true;
         for (clause const* cp : m_clauses) {
             clause const & c = *cp;
             if (!c.satisfied_by(m) && !c.is_blocked()) {
-                IF_VERBOSE(0, verbose_stream() << "model check failed: " << c << "\n";);
+                IF_VERBOSE(0, verbose_stream() << "failed clause " << c.id() << ": " << c << "\n";);
                 TRACE("sat", tout << "failed: " << c << "\n";
                       tout << "assumptions: " << m_assumptions << "\n";
                       tout << "trail: " << m_trail << "\n";
@@ -1612,10 +1615,16 @@ namespace sat {
                 ok = false;
             }
         }
+        return ok;
+    }
+
+    bool solver::check_model(model const & m) const {
+        bool ok = check_clauses(m);
         if (ok && !m_mc.check_model(m)) {
             ok = false;
             TRACE("sat", tout << "model: " << m << "\n"; m_mc.display(tout););
         }
+        IF_VERBOSE(1, verbose_stream() << "model check " << (ok?"OK":"failed") << "\n";);
         return ok;
     }
 
@@ -2034,7 +2043,7 @@ namespace sat {
                 process_antecedent(~(js.get_literal2()), num_marks);
                 break;
             case justification::CLAUSE: {
-                clause & c = *(m_cls_allocator.get_clause(js.get_clause_offset()));
+                clause & c = get_clause(js);
                 unsigned i   = 0;
                 if (consequent != null_literal) {
                     SASSERT(c[0] == consequent || c[1] == consequent);
@@ -2153,7 +2162,7 @@ namespace sat {
             process_antecedent_for_unsat_core(~(js.get_literal2()));
             break;
         case justification::CLAUSE: {
-            clause & c = *(m_cls_allocator.get_clause(js.get_clause_offset()));
+            clause & c = get_clause(js);
             unsigned i = 0;
             if (consequent != null_literal) {
                 SASSERT(c[0] == consequent || c[1] == consequent);
@@ -2497,7 +2506,7 @@ namespace sat {
                 }
                 break;
             case justification::CLAUSE: {
-                clause & c = *(m_cls_allocator.get_clause(js.get_clause_offset()));
+                clause & c = get_clause(js);
                 unsigned i   = 0;
                 if (c[0].var() == var) {
                     i = 1;
@@ -2624,8 +2633,7 @@ namespace sat {
         unsigned sz = m_lemma.size();
         SASSERT(!is_marked(m_lemma[0].var()));
         mark(m_lemma[0].var());
-        for (unsigned i = m_lemma.size(); i > 0; ) {
-            --i;
+        for (unsigned i = m_lemma.size(); i-- > 0; ) {
             justification js = m_justification[m_lemma[i].var()];
             switch (js.get_kind()) {
             case justification::NONE:
@@ -2638,9 +2646,9 @@ namespace sat {
                 update_lrb_reasoned(js.get_literal2());
                 break;
             case justification::CLAUSE: {
-                clause & c = *(m_cls_allocator.get_clause(js.get_clause_offset()));
-                for (unsigned i = 0; i < c.size(); ++i) {
-                    update_lrb_reasoned(c[i]);
+                clause & c = get_clause(js);
+                for (literal l : c) {
+                    update_lrb_reasoned(l);
                 }
                 break;
             }
@@ -3045,6 +3053,7 @@ namespace sat {
         m_scc.updt_params(p);
         m_rand.set_seed(m_config.m_random_seed);
         m_step_size = m_config.m_step_size_init;
+        m_drat.updt_config();
     }
 
     void solver::collect_param_descrs(param_descrs & d) {
@@ -3199,7 +3208,7 @@ namespace sat {
     std::ostream& solver::display_justification(std::ostream & out, justification const& js) const {
         out << js;
         if (js.is_clause()) {
-            out << *(m_cls_allocator.get_clause(js.get_clause_offset()));
+            out << get_clause(js);
         }
         else if (js.is_ext_justification() && m_ext) {
             m_ext->display_justification(out << " ", js.get_ext_justification_idx());
@@ -3845,11 +3854,11 @@ namespace sat {
             s |= m_antecedents.find(js.get_literal2().var());
             break;
         case justification::CLAUSE: {
-            clause & c = *(m_cls_allocator.get_clause(js.get_clause_offset()));
-            for (unsigned i = 0; i < c.size(); ++i) {
-                if (c[i] != lit) {
-                    if (check_domain(lit, ~c[i]) && all_found) {
-                        s |= m_antecedents.find(c[i].var());
+            clause & c = get_clause(js);
+            for (literal l : c) {
+                if (l != lit) {
+                    if (check_domain(lit, ~l) && all_found) {
+                        s |= m_antecedents.find(l.var());
                     }
                     else {
                         all_found = false;
diff --git a/src/sat/sat_solver.h b/src/sat/sat_solver.h
index 2c5a04104..e7dae93a2 100644
--- a/src/sat/sat_solver.h
+++ b/src/sat/sat_solver.h
@@ -366,6 +366,7 @@ namespace sat {
         model_converter const & get_model_converter() const { return m_mc; }
         void set_model(model const& mdl);
         char const* get_reason_unknown() const { return m_reason_unknown.c_str(); }
+        bool check_clauses(model const& m) const;
 
         literal select_lookahead(literal_vector const& assumptions, bool_var_vector const& vars);
         lbool  cube(bool_var_vector const& vars, literal_vector& lits);
@@ -442,6 +443,20 @@ namespace sat {
             justification const & jst = m_justification[l0.var()];
             return !jst.is_clause() || m_cls_allocator.get_clause(jst.get_clause_offset()) != &c;
         }
+
+        clause& get_clause(watch_list::iterator it) const {
+            SASSERT(it->get_kind() == watched::CLAUSE);
+            return get_clause(it->get_clause_offset());
+        }
+
+        clause& get_clause(justification const& j) const {
+            SASSERT(j.is_clause());
+            return get_clause(j.get_clause_offset());
+        }
+
+        clause& get_clause(clause_offset cls_off) const {
+            return *(m_cls_allocator.get_clause(cls_off));
+        }
         
         // -----------------------
         //
diff --git a/src/sat/sat_solver/inc_sat_solver.cpp b/src/sat/sat_solver/inc_sat_solver.cpp
index 742f9e103..5cf16ccc7 100644
--- a/src/sat/sat_solver/inc_sat_solver.cpp
+++ b/src/sat/sat_solver/inc_sat_solver.cpp
@@ -180,7 +180,13 @@ public:
         m_internalized_converted = false;
 
         init_reason_unknown();
-        r = m_solver.check(m_asms.size(), m_asms.c_ptr());
+        try {
+            r = m_solver.check(m_asms.size(), m_asms.c_ptr());
+        }
+        catch (z3_exception& ex) {
+            IF_VERBOSE(10, verbose_stream() << "exception: " << ex.msg() << "\n";);
+            r = l_undef;            
+        }
         if (r == l_undef && m_solver.get_config().m_dimacs_display) {
             for (auto const& kv : m_map) {
                 std::cout << "c " << kv.m_value << " " << mk_pp(kv.m_key, m) << "\n";
diff --git a/src/sat/tactic/goal2sat.cpp b/src/sat/tactic/goal2sat.cpp
index 208d60386..1270b67c2 100644
--- a/src/sat/tactic/goal2sat.cpp
+++ b/src/sat/tactic/goal2sat.cpp
@@ -896,6 +896,7 @@ struct sat2goal::imp {
     public:
         sat_model_converter(ast_manager & m, sat::solver const & s):m_var2expr(m) {
             m_mc.copy(s.get_model_converter());
+            m_mc.set_solver(&s);
             m_fmc = alloc(filter_model_converter, m);
         }