diff --git a/src/opt/dual_maxres.cpp b/src/opt/dual_maxres.cpp
index 3595149bf..292dc5e72 100644
--- a/src/opt/dual_maxres.cpp
+++ b/src/opt/dual_maxres.cpp
@@ -321,9 +321,12 @@ public:
     }
 
     lbool minimize_core(ptr_vector<expr>& core) {
+        if (m_sat_enabled) {
+            return l_true;
+        }
         m_mus.reset();
         for (unsigned i = 0; i < core.size(); ++i) {
-            m_mus.add_soft(core[i], 1, core.c_ptr() + i);
+            m_mus.add_soft(core[i]);
         }
         unsigned_vector mus_idx;
         lbool is_sat = m_mus.get_mus(mus_idx);
diff --git a/src/opt/maxres.cpp b/src/opt/maxres.cpp
index ecdd788a9..393d00f85 100644
--- a/src/opt/maxres.cpp
+++ b/src/opt/maxres.cpp
@@ -27,16 +27,9 @@ using namespace opt;
 
 
 class maxres : public maxsmt_solver_base {
-    struct info {
-        app*     m_cls;
-        rational m_weight;
-        info(app* cls, rational const& w):
-            m_cls(cls), m_weight(w) {}
-        info(): m_cls(0) {}
-    };
     expr_ref_vector  m_B;
     expr_ref_vector  m_asms;    
-    obj_map<expr, info> m_asm2info;
+    obj_map<expr, rational> m_asm2weight;
     ptr_vector<expr> m_new_core;
     mus              m_mus;
     expr_ref_vector  m_trail;
@@ -63,14 +56,12 @@ public:
         TRACE("opt", tout << mk_pp(e, m) << "\n";);
         expr_ref asum(m), fml(m);
         app_ref cls(m);
-        info inf(0,rational(0));
-        if (m_asm2info.find(e, inf)) {
-            inf.m_weight += w;
-            m_asm2info.insert(e, inf);
+        rational weight(0);
+        if (m_asm2weight.find(e, weight)) {
+            weight += w;
+            m_asm2weight.insert(e, weight);
             return;
         }
-        cls = mk_cls(e);
-        m_trail.push_back(cls);
         if (is_literal(e)) {
             asum = e;
         }
@@ -79,21 +70,18 @@ public:
             fml = m.mk_iff(asum, e);
             m_s->assert_expr(fml);
         }
-        new_assumption(asum, cls, w);
+        new_assumption(asum, w);
         m_upper += w;
     }
 
-    void new_assumption(expr* e, app* cls, rational const& w) {
+    void new_assumption(expr* e, rational const& w) {
         TRACE("opt", tout << "insert: " << mk_pp(e, m) << " : " << w << "\n";);
-        info inf(cls, w);
-        m_asm2info.insert(e, inf);
+        m_asm2weight.insert(e, w);
         m_asms.push_back(e);
         m_trail.push_back(e);        
     }
 
     lbool operator()() {
-        expr_ref fml(m);
-        ptr_vector<expr> core;
         solver::scoped_push _sc(*m_s.get());
         init();
         init_local();
@@ -125,28 +113,13 @@ public:
                 m_upper = m_lower;
                 return l_true;
             }
+            case l_false:
+                is_sat = process_unsat();
+                if (is_sat != l_true) return is_sat;
+                break;
             case l_undef:
                 return l_undef;
             default:
-                core.reset();
-                m_s->get_unsat_core(core);
-                TRACE("opt", display_vec(tout << "core: ", core.size(), core.c_ptr()););
-                SASSERT(!core.empty());
-                is_sat = minimize_core(core);
-                SASSERT(!core.empty());
-                if (core.empty()) {
-                    return l_false;
-                }
-                if (is_sat != l_true) {
-                    return is_sat;
-                }
-                remove_core(core);
-                rational w = split_core(core);
-                TRACE("opt", display_vec(tout << "minimized core: ", core.size(), core.c_ptr()););
-                max_resolve(core, w);
-                fml = m.mk_not(m.mk_and(m_B.size(), m_B.c_ptr()));
-                m_s->assert_expr(fml);
-                m_lower += w; 
                 break;
             }
             IF_VERBOSE(1, verbose_stream() << "(opt.max_res [" << m_lower << ":" << m_upper << "])\n";);
@@ -154,13 +127,78 @@ public:
         return l_true;
     }
 
+    lbool get_cores(vector<ptr_vector<expr> >& cores) {
+        // assume m_s is unsat.
+        lbool is_sat = l_false;
+        expr_ref_vector asms(m_asms);
+        cores.reset();
+        ptr_vector<expr> core;
+        while (is_sat == l_false) {
+            core.reset();
+            m_s->get_unsat_core(core);
+            is_sat = minimize_core(core);
+            if (is_sat != l_true) {
+                break;
+            }
+            cores.push_back(core);
+            // TBD: ad hoc to avoid searching for large cores..
+            if (core.size() >= 3) {
+                break;
+            }
+            remove_soft(core, asms);
+            TRACE("opt",
+                  display_vec(tout << "core: ", core.size(), core.c_ptr());
+                  display_vec(tout << "assumptions: ", asms.size(), asms.c_ptr()););
+            is_sat = m_s->check_sat(asms.size(), asms.c_ptr());            
+        }
+        TRACE("opt", 
+              tout << "num cores: " << cores.size() << "\n";
+              for (unsigned i = 0; i < cores.size(); ++i) {
+                  for (unsigned j = 0; j < cores[i].size(); ++j) {
+                      tout << mk_pp(cores[i][j], m) << " ";
+                  }
+                  tout << "\n";
+              }
+              tout << "num satisfying: " << asms.size() << "\n";);
+        
+        return is_sat;
+    }
+
+
+    lbool process_unsat() {
+        vector<ptr_vector<expr> > cores;
+        lbool is_sat = get_cores(cores);
+        if (is_sat != l_true) {
+            return is_sat;
+        }
+        if (cores.empty()) {
+            return l_false;
+        }
+        for (unsigned i = 0; is_sat == l_true && i < cores.size(); ++i) {
+            is_sat = process_unsat(cores[i]);
+        }
+        return is_sat;
+    }
+    
+    lbool process_unsat(ptr_vector<expr>& core) {
+        expr_ref fml(m);
+        remove_core(core);
+        rational w = split_core(core);
+        TRACE("opt", display_vec(tout << "minimized core: ", core.size(), core.c_ptr()););
+        max_resolve(core, w);
+        fml = m.mk_not(m.mk_and(m_B.size(), m_B.c_ptr()));
+        m_s->assert_expr(fml);
+        m_lower += w;
+        return l_true;
+    }
+
     lbool minimize_core(ptr_vector<expr>& core) {
+        if (m_sat_enabled) {
+            return l_true;
+        }
         m_mus.reset();
         for (unsigned i = 0; i < core.size(); ++i) {
-            app* cls = get_clause(core[i]);
-            SASSERT(cls);
-            SASSERT(m.is_or(cls));
-            m_mus.add_soft(core[i], cls->get_num_args(), cls->get_args());
+            m_mus.add_soft(core[i]);
         }
         unsigned_vector mus_idx;
         lbool is_sat = m_mus.get_mus(mus_idx);
@@ -177,11 +215,7 @@ public:
     }
 
     rational get_weight(expr* e) {
-        return m_asm2info.find(e).m_weight;
-    }
-
-    app* get_clause(expr* e) {
-        return m_asm2info.find(e).m_cls;
+        return m_asm2weight.find(e);
     }
 
     rational split_core(ptr_vector<expr> const& core) {
@@ -200,7 +234,7 @@ public:
             rational w2 = get_weight(core[i]);
             if (w2 > w) {
                 rational w3 = w2 - w;
-                new_assumption(core[i], get_clause(core[i]), w3);
+                new_assumption(core[i], w3);
             }
         }
         return w;
@@ -246,55 +280,25 @@ public:
             asum = mk_fresh_bool("a");
             cls = m.mk_or(b_i1, d);
             fml = m.mk_iff(asum, cls);
-            cls = mk_cls(cls);
-            m_trail.push_back(cls);
-            new_assumption(asum, cls, w);
+            new_assumption(asum, w);
             m_s->assert_expr(fml);
         }
     }
 
-    app_ref mk_cls(expr* e) {
-        expr_ref_vector disj(m), todo(m);
-        expr_ref f(m);
-        app_ref result(m);
-        expr* e1, *e2;
-        todo.push_back(e);
-        while (!todo.empty()) {
-            f = todo.back();
-            todo.pop_back();
-            if (m.is_implies(f, e1, e2)) {
-                todo.push_back(m.mk_not(e1));
-                todo.push_back(e2);
-            }
-            else if (m.is_not(f, e1) && m.is_not(e1, e2)) {
-                todo.push_back(e2);
-            }
-            else if (m.is_or(f)) {
-                todo.append(to_app(f)->get_num_args(), to_app(f)->get_args());
-            }
-            else if (m.is_not(f, e1) && m.is_and(e1)) {
-                for (unsigned i = 0; i < to_app(e1)->get_num_args(); ++i) {
-                    todo.push_back(m.mk_not(to_app(e1)->get_arg(i)));
-                }
-            }
-            else {
-                disj.push_back(f);
-            }
-        }
-        result = m.mk_or(disj.size(), disj.c_ptr());
-        return result;
-    }
-
-    void remove_core(ptr_vector<expr> const& core) {
-        for (unsigned i = 0; i < m_asms.size(); ++i) {
-            if (core.contains(m_asms[i].get())) {
-                m_asms[i] = m_asms.back();
-                m_asms.pop_back();
+    void remove_soft(ptr_vector<expr> const& core, expr_ref_vector& asms) {
+        for (unsigned i = 0; i < asms.size(); ++i) {
+            if (core.contains(asms[i].get())) {
+                asms[i] = asms.back();
+                asms.pop_back();
                 --i;
             }
         }
     }
 
+    void remove_core(ptr_vector<expr> const& core) {
+        remove_soft(core, m_asms);
+    }
+
     virtual void set_cancel(bool f) {
         maxsmt_solver_base::set_cancel(f);
         m_mus.set_cancel(f);
@@ -303,7 +307,6 @@ public:
     void init_local() {
         m_upper.reset();
         m_lower.reset();
-        m_asm2info.reset();
         m_trail.reset();
         for (unsigned i = 0; i < m_soft.size(); ++i) {
             add_soft(m_soft[i].get(), m_weights[i]);
diff --git a/src/opt/maxsmt.cpp b/src/opt/maxsmt.cpp
index 347f4bf3a..d183ab4fc 100644
--- a/src/opt/maxsmt.cpp
+++ b/src/opt/maxsmt.cpp
@@ -130,6 +130,7 @@ namespace opt {
     }
 
     void maxsmt_solver_base::enable_inc_bvsat() {
+        m_params.set_bool("minimize_core", true);
         solver* sat_solver = mk_inc_sat_solver(m, m_params);
         unsigned sz = s().get_num_assertions();
         for (unsigned i = 0; i < sz; ++i) {
@@ -138,25 +139,11 @@ namespace opt {
         m_s = sat_solver;
     }
 
-    void maxsmt_solver_base::enable_noninc_bvsat() {
-        tactic_ref pb2bv = mk_card2bv_tactic(m, m_params);
-        tactic_ref bv2sat = mk_qfbv_tactic(m, m_params);
-        tactic_ref tac = and_then(pb2bv.get(), bv2sat.get());
-        solver* sat_solver = mk_tactic2solver(m, tac.get(), m_params);
-        unsigned sz = s().get_num_assertions();
-        for (unsigned i = 0; i < sz; ++i) {
-            sat_solver->assert_expr(s().get_assertion(i));
-        }   
-        unsigned lvl = m_s->get_scope_level();
-        while (lvl > 0) { sat_solver->push(); --lvl; }
-        m_s = sat_solver;
-    }
 
 
     void maxsmt_solver_base::enable_bvsat()  {
         if (m_enable_sat && !m_sat_enabled && probe_bv()) {
             enable_inc_bvsat();
-            // enable_noninc_bvsat();
             m_sat_enabled = true;
         }
     }
diff --git a/src/opt/maxsmt.h b/src/opt/maxsmt.h
index f85f30d78..138670693 100644
--- a/src/opt/maxsmt.h
+++ b/src/opt/maxsmt.h
@@ -99,7 +99,6 @@ namespace opt {
         app* mk_fresh_bool(char const* name);
     private:
         void enable_inc_bvsat();
-        void enable_noninc_bvsat();
     };
 
     /**
diff --git a/src/opt/mus.cpp b/src/opt/mus.cpp
index 06d77aad9..5fd246294 100644
--- a/src/opt/mus.cpp
+++ b/src/opt/mus.cpp
@@ -7,7 +7,7 @@ Module Name:
 
 Abstract:
    
-    Faster MUS extraction based on Belov et.al. HYB (Algorithm 3, 4)
+    MUS extraction.
 
 Author:
 
@@ -35,58 +35,28 @@ struct mus::imp {
     ast_manager&                m;
     expr_ref_vector             m_cls2expr;
     obj_map<expr, unsigned>     m_expr2cls;
-    vector<smt::literal_vector> m_cls2lits;
-    expr_ref_vector             m_vars;
-    obj_map<expr, unsigned>     m_var2idx;
     volatile bool               m_cancel;
-    bool                        m_rmr_enabled;
 
     imp(ref<solver>& s, ast_manager& m): 
-        m_s(s), m(m), m_cls2expr(m), m_vars(m), m_cancel(false),
-        m_rmr_enabled(false) {}
+        m_s(s), m(m), m_cls2expr(m),  m_cancel(false)
+    {}
 
     void reset() {
         m_cls2expr.reset();
         m_expr2cls.reset();
-        m_cls2lits.reset();
-        m_vars.reset();
-        m_var2idx.reset();
-        m_vars.push_back(m.mk_true());
     }
         
     void set_cancel(bool f) {
         m_cancel = f;
     }
     
-    unsigned add_var(expr* v) {
-        unsigned idx = m_vars.size();
-        if (!m_var2idx.find(v, idx)) {
-            m_var2idx.insert(v, idx);
-            m_vars.push_back(v);
-        }
-        return idx;
-    }
     
-    unsigned add_soft(expr* cls, unsigned sz, expr* const* args) {
+    unsigned add_soft(expr* cls) {
         SASSERT(is_uninterp_const(cls) || m.is_not(cls) && is_uninterp_const(to_app(cls)->get_arg(0)));
-        smt::literal_vector lits;
-        expr* arg;
-        for (unsigned i = 0; i < sz; ++i) {
-            if (m.is_not(args[i], arg)) {
-                lits.push_back(smt::literal(add_var(arg), true));
-            }
-            else {
-                lits.push_back(smt::literal(add_var(args[i]), false));
-            }
-        }
-        unsigned idx = m_cls2lits.size();
+        unsigned idx = m_cls2expr.size();
         m_expr2cls.insert(cls, idx);
         m_cls2expr.push_back(cls);
-        m_cls2lits.push_back(lits);
-        TRACE("opt", 
-              tout << idx << ": " << mk_pp(cls, m) << "\n";
-              display_vec(tout, lits);
-              );
+        TRACE("opt", tout << idx << ": " << mk_pp(cls, m) << "\n";);
         return idx;
     }
 
@@ -99,11 +69,6 @@ struct mus::imp {
     
     lbool get_mus(unsigned_vector& mus) {
         // SASSERT: mus does not have duplicates.
-        TRACE("opt", 
-              for (unsigned i = 0; i < m_cls2lits.size(); ++i) {
-                  display_vec(tout, m_cls2lits[i]);
-              }
-              );
         unsigned_vector core;
         for (unsigned i = 0; i < m_cls2expr.size(); ++i) {
             core.push_back(i);
@@ -114,16 +79,13 @@ struct mus::imp {
         }
         mus.reset();
         expr_ref_vector assumptions(m);
-        svector<bool> model;
         ptr_vector<expr> core_exprs;
-        model.resize(m_vars.size());
         while (!core.empty()) { 
             IF_VERBOSE(1, verbose_stream() << "(opt.mus reducing core: " << core.size() << " new core: " << mus.size() << ")\n";);
             unsigned cls_id = core.back();
             TRACE("opt", 
                   display_vec(tout << "core:  ", core);
                   display_vec(tout << "mus:   ", mus);
-                  // display_vec(tout << "model: ", model);
                   );
             core.pop_back();
             expr* cls = m_cls2expr[cls_id].get();
@@ -140,13 +102,6 @@ struct mus::imp {
             case l_true:
                 assumptions.push_back(cls);
                 mus.push_back(cls_id);
-                if (m_rmr_enabled) {
-                    extract_model(model);
-                    sz = core.size();
-                    core.append(mus);
-                    rmr(core, mus, model);
-                    core.resize(sz);
-                }
                 break;
             default:
                 core_exprs.reset();
@@ -192,78 +147,6 @@ struct mus::imp {
         out << "\n";
     }
 
-    void extract_model(svector<bool>& model) {
-        model_ref mdl;
-        m_s->get_model(mdl);
-        for (unsigned i = 0; i < m_vars.size(); ++i) {
-            expr_ref tmp(m);
-            mdl->eval(m_vars[i].get(), tmp);
-            model[i] = m.is_true(tmp);
-        }
-        TRACE("opt", 
-              display_vec(tout << "model: ", model);
-              model_smt2_pp(tout, m, *mdl, 0);              
-              );
-
-    }
-
-    /**
-       Recursive model rotation.       
-    */
-    void rmr(unsigned_vector& M, unsigned_vector& mus, svector<bool>& model) {
-        TRACE("opt", 
-              display_vec(tout << "core:  ", M);
-              display_vec(tout << "mus:   ", mus);
-              display_vec(tout << "model: ", model););
-
-        unsigned cls_id = mus.back();
-        smt::literal_vector const& cls = m_cls2lits[cls_id];
-        for (unsigned i = 0; i < cls.size(); ++i) {
-            smt::literal lit = cls[i];
-            SASSERT(model[lit.var()] == lit.sign()); // literal evaluates to false.
-            model[lit.var()] = !model[lit.var()];    // swap assignment
-            if (has_single_unsat(model, cls_id) && 
-                !mus.contains(cls_id) && 
-                model_check(model, cls_id)) {
-                mus.push_back(cls_id);
-                rmr(M, mus, model);
-            }
-            model[lit.var()] = !model[lit.var()];    // swap assignment back            
-        }
-    }
-
-    bool model_check(svector<bool> const& model, unsigned cls_id) {
-        // model has to work for hard constraints.
-        return false;
-    }
-
-    bool has_single_unsat(svector<bool> const& model, unsigned& cls_id) const {
-        cls_id = UINT_MAX;
-        for (unsigned i = 0; i < m_cls2lits.size(); ++i) {
-            if (!eval(model, m_cls2lits[i])) {
-                if (cls_id == UINT_MAX) {
-                    cls_id = i;
-                }
-                else {
-                    return false;
-                }
-            }
-        }
-        TRACE("opt", display_vec(tout << "clause: " << cls_id << " model: ", model););
-        return cls_id != UINT_MAX;
-    }
-
-    bool eval(svector<bool> const& model, smt::literal_vector const& cls) const {
-        bool result = false;
-        for (unsigned i = 0; !result && i < cls.size(); ++i) {
-            result = (model[cls[i].var()] != cls[i].sign());
-        }
-        TRACE("opt", display_vec(tout << "model: ", model);
-              display_vec(tout << "clause: ", cls);
-              tout << "result: " << result << "\n";);
-        return result;
-    }
-
 };
 
 mus::mus(ref<solver>& s, ast_manager& m) {
@@ -274,8 +157,8 @@ mus::~mus() {
     dealloc(m_imp);
 }
 
-unsigned mus::add_soft(expr* cls, unsigned sz, expr* const* args) {
-    return m_imp->add_soft(cls, sz, args);
+unsigned mus::add_soft(expr* cls) {
+    return m_imp->add_soft(cls);
 }
 
 lbool mus::get_mus(unsigned_vector& mus) {
diff --git a/src/opt/mus.h b/src/opt/mus.h
index 064ab3b0e..e9e3b80bd 100644
--- a/src/opt/mus.h
+++ b/src/opt/mus.h
@@ -33,7 +33,7 @@ namespace opt {
            cls is equivalent to a disjunction of args.
            Assume also that cls is a literal.           
         */
-        unsigned add_soft(expr* cls, unsigned sz, expr* const* args);
+        unsigned add_soft(expr* cls);
         
         lbool get_mus(unsigned_vector& mus);
         
diff --git a/src/sat/sat_config.cpp b/src/sat/sat_config.cpp
index cc0f97ff0..94ac37de5 100644
--- a/src/sat/sat_config.cpp
+++ b/src/sat/sat_config.cpp
@@ -104,6 +104,7 @@ namespace sat {
             m_gc_increment    = p.gc_increment();
         }
         m_minimize_lemmas = p.minimize_lemmas();
+        m_minimize_core   = p.minimize_core();
         m_dyn_sub_res     = p.dyn_sub_res();
     }
 
diff --git a/src/sat/sat_config.h b/src/sat/sat_config.h
index 90ebe968b..47804793d 100644
--- a/src/sat/sat_config.h
+++ b/src/sat/sat_config.h
@@ -68,6 +68,8 @@ namespace sat {
 
         bool               m_minimize_lemmas;
         bool               m_dyn_sub_res;
+        bool               m_minimize_core;
+
 
         symbol             m_always_true;
         symbol             m_always_false;
diff --git a/src/sat/sat_mus.cpp b/src/sat/sat_mus.cpp
index 7000191fc..e7eecfe4a 100644
--- a/src/sat/sat_mus.cpp
+++ b/src/sat/sat_mus.cpp
@@ -15,8 +15,6 @@ Author:
 
 Notes:
 
-   Model rotation needs fixes to ensure that hard constraints are satisfied
-   under pertubed model. Model rotation also has o be consistent with theories.
 
 --*/
 
@@ -32,26 +30,23 @@ namespace sat {
     void mus::reset() {
         m_core.reset();
         m_mus.reset();
-        m_assumptions.reset();
     }
 
-    void mus::set_core() {
+    void mus::set_core() {        
         m_core.append(m_mus);                
         s.m_core.reset();
         s.m_core.append(m_core);
     }
 
     lbool mus::operator()() {
+        flet<bool> _disable_min(s.m_config.m_minimize_core, false);
+        TRACE("sat", tout << "old core: " << s.get_core() << "\n";);
+        IF_VERBOSE(2, verbose_stream() << "(sat.mus " << s.get_core() << ")\n";);
         reset();
         literal_vector& core = m_core;
         literal_vector& mus = m_mus;
-        literal_vector& assumptions = m_assumptions;
         core.append(s.get_core());
-        SASSERT(!core.empty());
 
-        if (core.size() == 1) {
-            return l_true;
-        }
         while (!core.empty()) {
             TRACE("sat", 
                   tout << "core: " << core << "\n";
@@ -63,29 +58,38 @@ namespace sat {
             }
             literal lit = core.back();
             core.pop_back();
-            unsigned sz = assumptions.size();
-            assumptions.push_back(~lit);
-            assumptions.append(core);
-            lbool is_sat = s.check(assumptions.size(), assumptions.c_ptr());
-            assumptions.resize(sz);
+            unsigned sz = mus.size();
+            //mus.push_back(~lit); // TBD: measure
+            mus.append(core);
+            lbool is_sat = s.check(mus.size(), mus.c_ptr());
+            mus.resize(sz);
             switch (is_sat) {
             case l_undef:
                 core.push_back(lit);
                 set_core();
                 return l_undef;
             case l_true: {
-                assumptions.push_back(lit);
+                SASSERT(value_at(lit, s.get_model()) == l_false);
                 mus.push_back(lit);
-                unsigned sz = core.size();
+                if (core.empty()) {
+                    break;
+                }
+                sz = core.size();
                 core.append(mus);
                 rmr();
                 core.resize(sz);
+                IF_VERBOSE(2, verbose_stream() << "(sat.mus.new " << mus << " " << core << ")\n";);
                 break;
             }
             case l_false:
+                literal_vector const& new_core = s.get_core();
+                if (new_core.contains(~lit)) {
+                    break;
+                }
+                IF_VERBOSE(2, verbose_stream() << "(sat.mus.new " << new_core << ")\n";);
                 core.reset();
-                for (unsigned i = 0; i < s.get_core().size(); ++i) {
-                    literal lit = s.get_core()[i];
+                for (unsigned i = 0; i < new_core.size(); ++i) {
+                    literal lit = new_core[i];
                     if (!mus.contains(lit)) {
                         core.push_back(lit);
                     }
@@ -93,22 +97,23 @@ namespace sat {
                 break;
             }
         }
+        TRACE("sat", tout << "new core: " << mus << "\n";);
+        set_core();
         return l_true;
     }
 
-    lbool mus::eval(literal l) const {
-        return value_at(l, s.get_model());
-    }
-
     void mus::rmr() {
         model& model = s.m_model;
         literal lit = m_mus.back();
         literal assumption_lit;
-        SASSERT(eval(lit) == l_false); // literal is false in current model.
+        SASSERT(value_at(lit, model) == l_false); 
+        // literal is false in current model.
+        unsigned sz = m_toswap.size();
         find_swappable(lit);
-        for (unsigned i = 0; i < m_toswap.size(); ++i) {
+        unsigned sz1 = m_toswap.size();
+        for (unsigned i = sz; i < sz1; ++i) {
             lit = m_toswap[i];
-            SASSERT(eval(lit) == l_false);  
+            SASSERT(value_at(lit, model) == l_false);  
             model[lit.var()] = ~model[lit.var()];    // swap assignment
             if (has_single_unsat(assumption_lit) && !m_mus.contains(assumption_lit)) {
                 m_mus.push_back(assumption_lit);
@@ -116,6 +121,7 @@ namespace sat {
             }
             model[lit.var()] = ~model[lit.var()];    // swap assignment back            
         }
+        m_toswap.resize(sz);
     }
 
     bool mus::has_single_unsat(literal& assumption_lit) {
@@ -123,41 +129,50 @@ namespace sat {
         return false;
     }
 
+    //
+    // lit is false in model.
+    // find clauses where ~lit occurs, and all other literals
+    // are false in model.
+    // for each of the probed literals, determine if swapping the
+    // assignment falsifies a hard clause, if not, add to m_toswap.
+    // 
+
     void mus::find_swappable(literal lit) {
-        m_toswap.reset();
+        IF_VERBOSE(2, verbose_stream() << "(sat.mus swap " << lit << ")\n";);
+        unsigned sz = m_toswap.size();
+        literal lit2, lit3;
+        model const& model = s.get_model();
+        SASSERT(value_at(lit, model) == l_false);  
+        watch_list const& wlist = s.get_wlist(lit);
+        watch_list::const_iterator it  = wlist.begin();
+        watch_list::const_iterator end = wlist.end();
+        for (; it != end; ++it) {
+            switch (it->get_kind()) {
+            case watched::BINARY:
+                lit2 = it->get_literal();
+                TRACE("sat", tout << ~lit << " " << lit2 << "\n";);
+                break;
+            case watched::TERNARY:
+                lit2 = it->get_literal1();
+                lit3 = it->get_literal2();
+                TRACE("sat", tout << ~lit << " " << lit2 << " " << lit3 << "\n";);
+                break;
+            case watched::CLAUSE: {
+                clause_offset cls_off = it->get_clause_offset();
+                clause& c = *(s.m_cls_allocator.get_clause(cls_off));
+                if (c.is_learned()) {
+                    break;
+                }
+                TRACE("sat", tout << c << "\n";);
+                break;
+            }
+            case watched::EXT_CONSTRAINT:
+                TRACE("sat", tout << "external constraint - should avoid rmr\n";);
+                m_toswap.resize(sz);
+                return;
+            }
+        }
     }
 
 }
 
-#if 0
-
-
-    bool has_single_unsat(svector<bool> const& model, unsigned& cls_id) const {
-        cls_id = UINT_MAX;
-        for (unsigned i = 0; i < m_cls2lits.size(); ++i) {
-            if (!eval(model, m_cls2lits[i])) {
-                if (cls_id == UINT_MAX) {
-                    cls_id = i;
-                }
-                else {
-                    return false;
-                }
-            }
-        }
-        TRACE("opt", display_vec(tout << "clause: " << cls_id << " model: ", model););
-        return cls_id != UINT_MAX;
-    }
-
-    bool eval(svector<bool> const& model, smt::literal_vector const& cls) const {
-        bool result = false;
-        for (unsigned i = 0; !result && i < cls.size(); ++i) {
-            result = (model[cls[i].var()] != cls[i].sign());
-        }
-        TRACE("opt", display_vec(tout << "model: ", model);
-              display_vec(tout << "clause: ", cls);
-              tout << "result: " << result << "\n";);
-        return result;
-    }
-
-
-#endif
diff --git a/src/sat/sat_mus.h b/src/sat/sat_mus.h
index 70ba4d21c..b2643ade6 100644
--- a/src/sat/sat_mus.h
+++ b/src/sat/sat_mus.h
@@ -22,7 +22,6 @@ Notes:
 namespace sat {
     class mus {
         literal_vector m_core;
-        literal_vector m_assumptions;
         literal_vector m_mus;
         literal_vector m_toswap;        
         solver& s;
@@ -31,12 +30,12 @@ namespace sat {
         ~mus();        
         lbool operator()();
     private:
+        lbool mus2();
         void rmr();
         bool has_single_unsat(literal& assumption_lit);
         void find_swappable(literal lit);
         void reset();
         void set_core();
-        lbool eval(literal l) const;
     };
 
 };
diff --git a/src/sat/sat_solver.cpp b/src/sat/sat_solver.cpp
index d0d3236c8..91dada236 100644
--- a/src/sat/sat_solver.cpp
+++ b/src/sat/sat_solver.cpp
@@ -46,8 +46,7 @@ namespace sat {
         m_case_split_queue(m_activity),
         m_qhead(0),
         m_scope_lvl(0),
-        m_params(p),
-        m_minimize_core(p.get_bool("minimize_core", false)) {
+        m_params(p) {
         m_config.updt_params(p);
     }
 
@@ -1045,7 +1044,7 @@ namespace sat {
         }
         if (!m_mc.check_model(m))
             ok = false;
-        TRACE("sat", tout << "checl: " << ok << "\n" << m << "\n";);
+        TRACE("sat", tout << "check: " << ok << "\n" << m << "\n";);
         return ok;
     }
 
@@ -1621,7 +1620,7 @@ namespace sat {
         }        
         while (idx > 0);
         reset_unmark(old_size);
-        if (m_minimize_core) {
+        if (m_config.m_minimize_core) {
             m_mus(); //ignore return value on cancelation.
         }
     }
@@ -2236,7 +2235,6 @@ namespace sat {
         m_probing.updt_params(p);
         m_scc.updt_params(p);
         m_rand.set_seed(p.get_uint("random_seed", 0));
-        m_minimize_core = p.get_bool("minimize_core", false);
     }
 
     void solver::collect_param_descrs(param_descrs & d) {
diff --git a/src/sat/sat_solver.h b/src/sat/sat_solver.h
index 91ecbc3fb..056567bf9 100644
--- a/src/sat/sat_solver.h
+++ b/src/sat/sat_solver.h
@@ -123,7 +123,6 @@ namespace sat {
         literal_vector          m_assumptions;
         literal_set             m_assumption_set;
         literal_vector          m_core;
-        bool                    m_minimize_core;
 
         void del_clauses(clause * const * begin, clause * const * end);