diff --git a/contrib/cmake/src/test/CMakeLists.txt b/contrib/cmake/src/test/CMakeLists.txt
index 6ea07e84c..7ea5f4589 100644
--- a/contrib/cmake/src/test/CMakeLists.txt
+++ b/contrib/cmake/src/test/CMakeLists.txt
@@ -92,6 +92,7 @@ add_executable(test-z3
   rational.cpp
   rcf.cpp
   region.cpp
+  sat_lookahead.cpp
   sat_user_scope.cpp
   simple_parser.cpp
   simplex.cpp
diff --git a/src/sat/card_extension.cpp b/src/sat/card_extension.cpp
index f6bff3cd5..8fd0d89a8 100644
--- a/src/sat/card_extension.cpp
+++ b/src/sat/card_extension.cpp
@@ -709,6 +709,37 @@ namespace sat {
         return result;
     }
 
+    void card_extension::find_mutexes(literal_vector& lits, vector<literal_vector> & mutexes) {
+        literal_set slits(lits);
+        bool change = false;
+        for (unsigned i = 0; i < m_constraints.size(); ++i) {
+            card& c = *m_constraints[i];
+            if (c.size() == c.k() + 1) {
+                literal_vector mux;
+                for (unsigned j = 0; j < c.size(); ++j) {
+                    literal lit = ~c[j];
+                    if (slits.contains(lit)) {
+                        mux.push_back(lit);
+                    }
+                }
+                if (mux.size() <= 1) {
+                    continue;
+                }
+
+                for (unsigned j = 0; j < mux.size(); ++j) {
+                    slits.remove(mux[j]);
+                }
+                change = true;
+                mutexes.push_back(mux);
+            }
+        }        
+        if (!change) return;
+        literal_set::iterator it = slits.begin(), end = slits.end();
+        lits.reset();
+        for (; it != end; ++it) {
+            lits.push_back(*it);
+        }
+    }
 
     void card_extension::display_watch(std::ostream& out, bool_var v, bool sign) const {
         watch const* w = m_var_infos[v].m_lit_watch[sign];
@@ -957,7 +988,6 @@ namespace sat {
                tout << lits << "\n";);
         return value < p.m_k;
     }
-
     
 };
 
diff --git a/src/sat/card_extension.h b/src/sat/card_extension.h
index 3b65afb6b..65a991ec2 100644
--- a/src/sat/card_extension.h
+++ b/src/sat/card_extension.h
@@ -163,6 +163,7 @@ namespace sat {
         virtual std::ostream& display_justification(std::ostream& out, ext_justification_idx idx) const;
         virtual void collect_statistics(statistics& st) const;
         virtual extension* copy(solver* s);
+        virtual void find_mutexes(literal_vector& lits, vector<literal_vector> & mutexes);
     };
 
 };
diff --git a/src/sat/sat_config.cpp b/src/sat/sat_config.cpp
index cd756d79b..e5fac42ae 100644
--- a/src/sat/sat_config.cpp
+++ b/src/sat/sat_config.cpp
@@ -113,9 +113,9 @@ namespace sat {
         m_minimize_lemmas = p.minimize_lemmas();
         m_core_minimize   = p.core_minimize();
         m_core_minimize_partial   = p.core_minimize_partial();
-        m_drat            = p.drat() && p.threads() == 1;
         m_drat_check      = p.drat_check();
         m_drat_file       = p.drat_file();
+        m_drat            = (m_drat_check || m_drat_file != symbol("")) && p.threads() == 1;
         m_dyn_sub_res     = p.dyn_sub_res();
     }
 
diff --git a/src/sat/sat_elim_eqs.cpp b/src/sat/sat_elim_eqs.cpp
index 8cde147cc..36dc3a8d9 100644
--- a/src/sat/sat_elim_eqs.cpp
+++ b/src/sat/sat_elim_eqs.cpp
@@ -151,8 +151,6 @@ namespace sat {
                 break;
             default:
                 SASSERT(*it == &c);
-                *it2 = *it;
-                it2++;
                 if (j < sz) {
                     if (m_solver.m_config.m_drat) m_solver.m_drat.del(c); 
                     c.shrink(j);
@@ -162,10 +160,12 @@ namespace sat {
                     c.update_approx();
 
                 DEBUG_CODE({
-                        for (unsigned i = 0; i < sz; i++) {
+                        for (unsigned i = 0; i < j; i++) {
                             SASSERT(c[i] == norm(roots, c[i]));
                         } });
                 
+                *it2 = *it;
+                it2++;
                 if (!c.frozen()) {
                     m_solver.attach_clause(c);
                 }
diff --git a/src/sat/sat_extension.h b/src/sat/sat_extension.h
index 042f68e24..aae99c28f 100644
--- a/src/sat/sat_extension.h
+++ b/src/sat/sat_extension.h
@@ -47,6 +47,7 @@ namespace sat {
         virtual std::ostream& display_justification(std::ostream& out, ext_justification_idx idx) const = 0;
         virtual void collect_statistics(statistics& st) const = 0;
         virtual extension* copy(solver* s) = 0;
+        virtual void find_mutexes(literal_vector& lits, vector<literal_vector> & mutexes) = 0;
     };
 
 };
diff --git a/src/sat/sat_lookahead.h b/src/sat/sat_lookahead.h
new file mode 100644
index 000000000..f70ceab38
--- /dev/null
+++ b/src/sat/sat_lookahead.h
@@ -0,0 +1,394 @@
+/*++
+Copyright (c) 2017 Microsoft Corporation
+
+Module Name:
+
+    sat_lookahead.h
+
+Abstract:
+   
+    Lookahead SAT solver in the style of March.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2017-2-11
+
+Notes:
+
+--*/
+#ifndef _SAT_LOOKAHEAD_H_
+#define _SAT_LOOKAHEAD_H_
+
+namespace sat {
+    class lookahead {
+        solver& s;
+
+        struct config {
+            double   m_dl_success;
+        };
+
+        config          m_config;
+        double          m_delta_trigger; 
+        literal_vector  m_trail;
+        literal_vector  m_units;
+        unsigned_vector m_units_lim;
+        unsigned_vector m_learned_lim;
+
+
+        void init() {
+            m_delta_trigger = s.num_vars()/10;
+            m_config.m_dl_success = 0.8;
+        }
+        
+        void push(literal lit) { 
+            m_learned_lim.push_back(s.m_learned.size());
+            m_units_lim.push_back(m_units.size());
+            m_trail.push_back(lit);
+            s.push();
+            assign(lit);
+        }
+
+        void pop() { 
+            s.pop(1); 
+            unsigned old_sz = m_learned_lim.back();
+            m_learned_lim.pop_back();
+            for (unsigned i = old_sz; i < s.m_learned.size(); ++i) {
+                clause* r = s.m_learned[i];
+                s.dettach_clause(*r);
+                s.m_cls_allocator.del_clause(r);
+            }
+            s.m_learned.shrink(old_sz);
+            literal lits[2] = { m_trail.back(), null_literal };
+            unsigned new_unit_sz = m_units_lim.back();
+            for (unsigned i = m_units.size(); i > new_unit_sz; ) {
+                --i;
+                lits[1] = m_units[i];
+                clause * r = s.m_cls_allocator.mk_clause(2, lits, true);
+                s.m_learned.push_back(r);
+            }
+            m_units.shrink(new_unit_sz);
+            m_units_lim.pop_back();
+            m_trail.pop_back();
+        }
+        
+        unsigned diff(unsigned value0) const {
+            unsigned value1 = get_state_value();
+            SASSERT(value1 >= value0);
+            return value1 - value0;
+        }
+
+        unsigned mix_diff(unsigned l, unsigned r) const {
+            return l + r + (1 << 10) * l * r;
+        }
+
+        void get_resolvent_units(literal lit) {
+            if (inconsistent()) return;
+            for (unsigned i = s.m_trail.size(); i > 0; ) {
+                --i;
+                literal l = s.m_trail[i];
+                if (l == lit) break;
+                SASSERT(s.lvl(l) == s.scope_lvl());
+                watch_list& wlist = s.m_watches[(~l).index()];
+                watch_list::iterator it = wlist.begin(), end = wlist.end();
+                for (; it != end; ++it) {
+                    switch (it->get_kind()) {
+                    case watched::TERNARY:
+                        if (s.value(it->get_literal1()) == l_false && 
+                            s.value(it->get_literal()) == l_false) {
+                            m_units.push_back(l);
+                            goto done_watch_list;
+                        }
+                        break;
+                    case watched::CLAUSE: {
+                        clause_offset cls_off = it->get_clause_offset();
+                        clause & c = *(s.m_cls_allocator.get_clause(cls_off));
+                        if (c.size() == 2) break;
+                        SASSERT(c.size() > 2);
+                        if (c[0] == l && s.value(c[1]) == l_false) {
+                            DEBUG_CODE(for (unsigned j = 2; j < c.size(); ++j) SASSERT(s.value(c[j]) == l_false););
+                            m_units.push_back(l);
+                            goto done_watch_list;
+                        } 
+                        if (c[1] == l && s.value(c[0]) == l_false) {
+                            DEBUG_CODE(for (unsigned j = 2; j < c.size(); ++j) SASSERT(s.value(c[j]) == l_false););
+                            m_units.push_back(l);
+                            goto done_watch_list;
+                        } 
+                        break;
+                    }
+                    default:
+                        break;
+                    }
+                }
+            done_watch_list:
+                continue;
+            }
+        }
+
+        literal choose() {
+            literal l;
+            while (!choose1(l)) {};
+            return l;
+        }
+
+        unsigned get_state_value() const {
+            return s.m_learned.size();
+        }
+
+        bool choose1(literal& l) {
+            literal_vector P;
+            pre_select(P);
+            l = null_literal;
+            if (P.empty()) {
+                return true;
+            }
+            unsigned value0 = get_state_value();
+            unsigned h = 0, count = 1;
+            literal_vector& units = m_units;;
+            for (unsigned i = 0; i < P.size(); ++i) {
+                literal lit = P[i];
+
+                push(lit);
+                if (do_double(value0)) double_look();
+                if (inconsistent()) {
+                    pop();
+                    assign(~lit);
+                    if (do_double(value0)) double_look();
+                    if (inconsistent()) return true;
+                    continue;
+                }
+                unsigned diff1 = diff(value0);
+                pop();
+                
+                push(~lit);
+                if (do_double(value0)) double_look();
+                bool unsat2 = inconsistent();
+                unsigned diff2 = diff(value0);
+                pop();
+
+                if (unsat2) {
+                    assign(lit);
+                    continue;
+                }
+
+                unsigned mixd = mix_diff(diff1, diff2);
+
+                if (mixd > h || (mixd == h && s.m_rand(count) == 0)) {
+                    h = mixd;
+                    l = diff1 < diff2 ? lit : ~lit;
+                    ++count;
+                }
+            }
+            return l != null_literal;
+        }
+
+        void double_look() {
+            literal_vector P;
+            pre_select(P);
+            for (unsigned i = 0; !inconsistent() && i < P.size(); ++i) {
+                literal lit = P[i];
+
+                push(lit);
+                bool unsat = inconsistent();
+                pop();
+                if (unsat) {
+                    assign(~lit);
+                    continue;
+                }
+
+                push(~lit);
+                unsat = inconsistent();
+                pop();
+                if (unsat) {
+                    assign(lit);
+                }
+
+            }
+            update_delta_trigger();
+        }
+
+        void assign(literal l) {
+            s.assign(l, justification());
+            s.propagate(false);
+            get_resolvent_units(l);
+        }
+
+        bool inconsistent() { return s.inconsistent(); }
+
+        void pre_select(literal_vector& P) {
+            select_variables(P);
+            order_by_implication_trees(P);            
+        }
+
+        void order_by_implication_trees(literal_vector& P) {
+            literal_set roots;
+            literal_vector nodes, parent;
+
+            //
+            // Extract binary clauses in watch list. 
+            // Produce implication graph between literals in P.
+            //
+            
+            for (unsigned i = 0; i < P.size(); ++i) {
+                literal lit1 = P[i], lit2;
+
+                //
+                // lit2 => lit1, where lit2 is a root.
+                // make lit1 a root instead of lit2
+                //
+
+                watch_list& wlist = s.m_watches[(~lit1).index()];
+                watch_list::iterator it = wlist.begin(), end = wlist.end();
+                lit2 = null_literal;
+                for (; it != end; ++it) {
+                    switch (it->get_kind()) {
+                    case watched::BINARY:
+                        lit2 = it->get_literal();
+                        break;
+                    case watched::CLAUSE: {
+                        clause_offset cls_off = it->get_clause_offset();
+                        clause & c = *(s.m_cls_allocator.get_clause(cls_off));
+                        if (c.size() == 2) {
+                            if (c[0] == ~lit1) {
+                                lit2 = c[1];
+                            }
+                            else {
+                                SASSERT(c[1] == ~lit1);
+                                lit2 = c[0];
+                            }
+                        }
+                        break;
+                    }
+                    default:
+                        break;
+                    }
+                    
+                    if (lit2 != null_literal && roots.contains(lit2)) {
+                        // lit2 => lit1
+                        // if lit2 is a root, put it under lit2
+                        parent.setx(lit2.index(), lit1, null_literal);
+                        roots.remove(lit2);
+                        roots.insert(lit1);
+                        goto found;
+                    }
+                }
+
+                //
+                // lit1 => lit2.
+                // if lit2 is a node, put lit1 above lit2
+                //
+
+                it  = s.m_watches[lit1.index()].begin();
+                end = s.m_watches[lit1.index()].end();
+                for (; it != end; ++it) {
+                    lit2 = null_literal;
+                    switch (it->get_kind()) {
+                    case watched::BINARY:
+                        lit2 = it->get_literal();
+                        break;
+                    case watched::CLAUSE: {
+                        clause_offset cls_off = it->get_clause_offset();
+                        clause & c = *(s.m_cls_allocator.get_clause(cls_off));
+                        if (c.size() == 2) {
+                            if (c[0] == lit1) {
+                                lit2 = c[1];
+                            }
+                            else {
+                                SASSERT(c[1] == lit1);
+                                lit2 = c[0];
+                            }
+                        }
+                        break;
+                    }
+                    default:
+                        break;
+                    }
+                    
+                    if (lit2 != null_literal && nodes.contains(lit2)) {
+                        // lit1 => lit2
+                        parent.setx(lit1.index(), lit2, null_literal);
+                        nodes.insert(lit1);
+                        goto found;
+                    }
+                }
+                std::cout << "no parents or children of literal " << lit1 << "\n";
+                nodes.push_back(lit1);
+                roots.insert(lit1);                
+            found:
+                ;
+            }       
+            std::cout << "implication trees\n";
+            for (unsigned i = 0; i < parent.size(); ++i) {
+                literal p = parent[i];
+                if (p != null_literal) {
+                    std::cout << to_literal(i) << " |-> " << p << "\n";
+                }
+            }
+            
+        }
+
+        void select_variables(literal_vector& P) {
+            for (unsigned i = 0; i < s.num_vars(); ++i) {
+                if (s.value(i) == l_undef) {
+                    P.push_back(literal(i, false));
+                }
+            }
+        }
+
+        bool do_double(unsigned value0) {
+            unsigned value1 = get_state_value();
+            return !inconsistent() && value1 - value0 > m_delta_trigger;
+        }
+
+        void update_delta_trigger() {
+            if (inconsistent()) {
+                m_delta_trigger -= (1 - m_config.m_dl_success) / m_config.m_dl_success; 
+            }
+            else {
+                m_delta_trigger += 1;
+            }
+            if (m_delta_trigger >= s.num_vars()) {
+                // reset it.
+            }
+        }
+
+        lbool search() {
+            literal_vector trail;
+
+#define BACKTRACK                                       \
+            if (inconsistent()) {                       \
+                if (trail.empty()) return l_false;      \
+                pop();                                  \
+                assign(~trail.back());                  \
+                trail.pop_back();                       \
+                continue;                               \
+            }                                           \
+
+            while (true) {
+                s.checkpoint();
+                BACKTRACK;
+                literal l = choose();
+                TRACE("sat", tout << l << "\n";);
+                BACKTRACK;
+                if (l == null_literal) {
+                    return l_true;
+                }
+                push(l);
+                trail.push_back(l);
+            }
+        }
+
+    public:
+        lookahead(solver& s) : s(s) {
+            init();
+        }
+        
+        lbool check() {
+            return search();
+        }
+              
+    };
+}
+
+#endif
+
diff --git a/src/sat/sat_parallel.cpp b/src/sat/sat_parallel.cpp
index 493e9fe18..bc7ac408e 100644
--- a/src/sat/sat_parallel.cpp
+++ b/src/sat/sat_parallel.cpp
@@ -185,12 +185,17 @@ namespace sat {
         unsigned owner = s.m_par_id;
         while (m_pool.get_vector(owner, n, ptr)) {
             m_lits.reset();
-            for (unsigned i = 0; i < n; ++i) {
-                m_lits.push_back(to_literal(ptr[i]));
+            bool usable_clause = true;
+            for (unsigned i = 0; usable_clause && i < n; ++i) {
+                literal lit(to_literal(ptr[i]));                
+                m_lits.push_back(lit);
+                usable_clause = lit.var() <= s.m_par_num_vars && !s.was_eliminated(lit.var());
             }
             IF_VERBOSE(3, verbose_stream() << s.m_par_id << ": retrieve " << m_lits << "\n";);
             SASSERT(n >= 2);
-            s.mk_clause_core(m_lits.size(), m_lits.c_ptr(), true);
+            if (usable_clause) {
+                s.mk_clause_core(m_lits.size(), m_lits.c_ptr(), true);
+            }
         }        
     }
 
diff --git a/src/sat/sat_params.pyg b/src/sat/sat_params.pyg
index 4a013a7fd..3413d5e6a 100644
--- a/src/sat/sat_params.pyg
+++ b/src/sat/sat_params.pyg
@@ -24,7 +24,6 @@ def_module_params('sat',
                           ('core.minimize_partial', BOOL, False, 'apply partial (cheap) core minimization'),
                           ('threads', UINT, 1, 'number of parallel threads to use'),
                           ('dimacs.core', BOOL, False, 'extract core from DIMACS benchmarks'),
-                          ('drat', BOOL, False, 'produce DRAT proofs'),
                           ('drat.file', SYMBOL, '', 'file to dump DRAT proofs'),
                           ('drat.check', BOOL, False, 'build up internal proof and check'),
                        ))
diff --git a/src/sat/sat_simplifier.cpp b/src/sat/sat_simplifier.cpp
index 3c4d784cb..03ff7ec98 100644
--- a/src/sat/sat_simplifier.cpp
+++ b/src/sat/sat_simplifier.cpp
@@ -35,6 +35,10 @@ namespace sat {
     void use_list::insert(clause & c) {
         unsigned sz = c.size();
         for (unsigned i = 0; i < sz; i++) {
+            if (m_use_list.size() <= c[i].index()) {
+                std::cout << c[i] << "\n";
+                std::cout << m_use_list.size() << "\n";
+            }
             m_use_list[c[i].index()].insert(c);
         }
     }
diff --git a/src/sat/sat_solver.cpp b/src/sat/sat_solver.cpp
index 3b96fa098..476295f7a 100644
--- a/src/sat/sat_solver.cpp
+++ b/src/sat/sat_solver.cpp
@@ -836,7 +836,7 @@ namespace sat {
         int num_threads = static_cast<int>(m_config.m_num_threads);
         int num_extra_solvers = num_threads - 1;
         sat::parallel par(*this);
-        par.reserve(num_threads, 1 << 9);
+        par.reserve(num_threads, 1 << 12);
         par.init_solvers(*this, num_extra_solvers);
         int finished_id = -1;
         std::string        ex_msg;
@@ -3076,17 +3076,21 @@ namespace sat {
             }
         }
         vector<unsigned_vector> _mutexes;
+        literal_vector _lits(lits);
+        if (m_ext) {
+            m_ext->find_mutexes(_lits, mutexes);
+        }
         unsigned_vector ps;
-        for (unsigned i = 0; i < lits.size(); ++i) {
-            ps.push_back(lits[i].index());
+        for (unsigned i = 0; i < _lits.size(); ++i) {
+            ps.push_back(_lits[i].index());
         }
         mc.cliques(ps, _mutexes);
         for (unsigned i = 0; i < _mutexes.size(); ++i) {
-            literal_vector lits;
+            literal_vector clique;
             for (unsigned j = 0; j < _mutexes[i].size(); ++j) {
-                lits.push_back(to_literal(_mutexes[i][j]));
+                clique.push_back(to_literal(_mutexes[i][j]));
             }
-            mutexes.push_back(lits);
+            mutexes.push_back(clique);
         }
         return l_true;
     }
diff --git a/src/sat/sat_solver.h b/src/sat/sat_solver.h
index 30d4ca4ad..37e5f1bb6 100644
--- a/src/sat/sat_solver.h
+++ b/src/sat/sat_solver.h
@@ -154,6 +154,7 @@ namespace sat {
         friend class drat;
         friend class card_extension;
         friend class parallel;
+        friend class lookahead;
         friend struct mk_stat;
     public:
         solver(params_ref const & p, reslimit& l);
diff --git a/src/test/main.cpp b/src/test/main.cpp
index 9239d0119..18b83e90e 100644
--- a/src/test/main.cpp
+++ b/src/test/main.cpp
@@ -229,6 +229,7 @@ int main(int argc, char ** argv) {
     TST(model_evaluator);
     TST(get_consequences);
     TST(pb2bv);
+    TST_ARGV(sat_lookahead);
     //TST_ARGV(hs);
 }