diff --git a/src/api/api_ast.cpp b/src/api/api_ast.cpp
index 9955cbc8f..eda6831b5 100644
--- a/src/api/api_ast.cpp
+++ b/src/api/api_ast.cpp
@@ -964,8 +964,7 @@ extern "C" {
             case PR_TH_LEMMA: return Z3_OP_PR_TH_LEMMA;
             case PR_HYPER_RESOLVE: return Z3_OP_PR_HYPER_RESOLVE;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
         if (mk_c(c)->get_arith_fid() == _d->get_family_id()) {
@@ -989,8 +988,7 @@ extern "C" {
             case OP_TO_INT: return Z3_OP_TO_INT;
             case OP_IS_INT: return Z3_OP_IS_INT;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
         if (mk_c(c)->get_array_fid() == _d->get_family_id()) {
@@ -1008,8 +1006,7 @@ extern "C" {
             case OP_AS_ARRAY: return Z3_OP_AS_ARRAY;
             case OP_ARRAY_EXT: return Z3_OP_ARRAY_EXT;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
 
@@ -1075,8 +1072,7 @@ extern "C" {
             case OP_BUREM_I: return Z3_OP_BUREM_I;
             case OP_BSMOD_I: return Z3_OP_BSMOD_I;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
         if (mk_c(c)->get_dt_fid() == _d->get_family_id()) {
@@ -1086,8 +1082,7 @@ extern "C" {
             case OP_DT_ACCESSOR:     return Z3_OP_DT_ACCESSOR;
             case OP_DT_UPDATE_FIELD: return Z3_OP_DT_UPDATE_FIELD;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
         if (mk_c(c)->get_datalog_fid() == _d->get_family_id()) {
@@ -1108,8 +1103,7 @@ extern "C" {
             case datalog::OP_DL_CONSTANT: return Z3_OP_FD_CONSTANT;
             case datalog::OP_DL_LT: return Z3_OP_FD_LT;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
 
@@ -1135,7 +1129,7 @@ extern "C" {
             case Z3_OP_RE_CONCAT: return Z3_OP_RE_CONCAT;
             case Z3_OP_RE_UNION: return Z3_OP_RE_UNION;
             default:
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
 
@@ -1195,8 +1189,7 @@ extern "C" {
             case OP_FPA_INTERNAL_TO_IEEE_BV_UNSPECIFIED:
                 return Z3_OP_UNINTERPRETED;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
 
@@ -1205,8 +1198,7 @@ extern "C" {
             case OP_LABEL: return Z3_OP_LABEL;
             case OP_LABEL_LIT: return Z3_OP_LABEL_LIT;
             default:
-                UNREACHABLE();
-                return Z3_OP_UNINTERPRETED;
+                return Z3_OP_INTERNAL;
             }
         }
 
@@ -1215,7 +1207,7 @@ extern "C" {
             case OP_PB_LE: return Z3_OP_PB_LE;
             case OP_PB_GE: return Z3_OP_PB_GE;
             case OP_AT_MOST_K: return Z3_OP_PB_AT_MOST;
-            default: UNREACHABLE();
+            default: return Z3_OP_INTERNAL;
             }
         }
 
diff --git a/src/api/z3_api.h b/src/api/z3_api.h
index 5ce2740d9..41ca923bb 100644
--- a/src/api/z3_api.h
+++ b/src/api/z3_api.h
@@ -945,6 +945,8 @@ typedef enum
 
       - Z3_OP_FPA_TO_IEEE_BV: Floating-point conversion to IEEE-754 bit-vector
 
+      - Z3_OP_INTERNAL: internal (often interpreted) symbol, but no additional information is exposed. Tools may use the string representation of the function declaration to obtain more information.
+
       - Z3_OP_UNINTERPRETED: kind used for uninterpreted symbols.
 */
 typedef enum {
@@ -1217,6 +1219,8 @@ typedef enum {
     Z3_OP_FPA_MIN_I,
     Z3_OP_FPA_MAX_I,
 
+    Z3_OP_INTERNAL,
+
     Z3_OP_UNINTERPRETED
 } Z3_decl_kind;
 
diff --git a/src/ast/ast.cpp b/src/ast/ast.cpp
index d8a6c47bc..d138685ed 100644
--- a/src/ast/ast.cpp
+++ b/src/ast/ast.cpp
@@ -2991,7 +2991,10 @@ proof * ast_manager::mk_unit_resolution(unsigned num_proofs, proof * const * pro
           for (unsigned i = 0; i < num_proofs; i++) tout << mk_pp(get_fact(proofs[i]), *this) << "\n";
           tout << "===>\n";
           tout << mk_pp(new_fact, *this) << "\n";);
-        SASSERT(num_proofs == cls_sz || (num_proofs == cls_sz + 1 && is_false(new_fact)));
+        //
+        // typically: num_proofs == cls_sz || (num_proofs == cls_sz + 1 && is_false(new_fact))
+        // but formula could have repeated literals that are merged in the clausal representation.
+        //
         unsigned num_matches = 0;
         for (unsigned i = 0; i < cls_sz; i++) {
             expr * lit = cls->get_arg(i);
diff --git a/src/smt/smt_internalizer.cpp b/src/smt/smt_internalizer.cpp
index 45bd75ab7..a17271e4e 100644
--- a/src/smt/smt_internalizer.cpp
+++ b/src/smt/smt_internalizer.cpp
@@ -1045,15 +1045,13 @@ namespace smt {
     bool context::simplify_aux_clause_literals(unsigned & num_lits, literal * lits, literal_buffer & simp_lits) {
         std::sort(lits, lits + num_lits);
         literal prev = null_literal;
-        unsigned i = 0;
         unsigned j = 0;
-        for (; i < num_lits; i++) {
+        for (unsigned i = 0; i < num_lits; i++) {
             literal curr = lits[i];
             lbool   val  = get_assignment(curr);
-            if (val == l_false)
-                simp_lits.push_back(~curr);
             switch(val) {
             case l_false:
+                simp_lits.push_back(~curr);
                 break; // ignore literal
             case l_undef:
                 if (curr == ~prev)
@@ -1295,8 +1293,9 @@ namespace smt {
                     SASSERT(get_assignment(simp_lits[i]) == l_true);
                 }
             });
-            if (old_num_lits != num_lits) 
+            if (!simp_lits.empty()) {
                 j = mk_justification(unit_resolution_justification(m_region, j, simp_lits.size(), simp_lits.c_ptr()));
+            }
             break;
         }
         case CLS_AUX_LEMMA: {
diff --git a/src/smt/smt_justification.cpp b/src/smt/smt_justification.cpp
index de3594fcc..c091f6973 100644
--- a/src/smt/smt_justification.cpp
+++ b/src/smt/smt_justification.cpp
@@ -52,6 +52,7 @@ namespace smt {
                   tout << lits[i] << " ";
               }
               tout << "\n";);
+        SASSERT(m_num_literals > 0);
     }
 
     unit_resolution_justification::unit_resolution_justification(justification * js, 
@@ -68,6 +69,7 @@ namespace smt {
                   tout << lits[i] << " ";
               }
               tout << "\n";);
+        SASSERT(num_lits != 0);
     }
 
     unit_resolution_justification::~unit_resolution_justification() {
diff --git a/src/smt/theory_bv.cpp b/src/smt/theory_bv.cpp
index 2945c8c93..a53580b73 100644
--- a/src/smt/theory_bv.cpp
+++ b/src/smt/theory_bv.cpp
@@ -312,6 +312,7 @@ namespace smt {
         SASSERT(v != null_theory_var);
         unsigned sz             = bits.size();
         SASSERT(get_bv_size(n) == sz);
+        m_bits[v].reset();
         for (unsigned i = 0; i < sz; i++) {
             expr * bit          = bits.get(i);
             expr_ref s_bit(m);
@@ -809,6 +810,7 @@ namespace smt {
         theory_var v       = e->get_th_var(get_id());
         unsigned num_args  = n->get_num_args();
         unsigned i         = num_args;
+        m_bits[v].reset();
         while (i > 0) {
             i--;
             theory_var arg = get_arg_var(e, i);
@@ -830,6 +832,7 @@ namespace smt {
         unsigned end       = n->get_decl()->get_parameter(0).get_int();
         SASSERT(start <= end);
         literal_vector & arg_bits = m_bits[arg];
+        m_bits[v].reset();
         for (unsigned i = start; i <= end; ++i)
             add_bit(v, arg_bits[i]);
         find_wpos(v);
@@ -1533,6 +1536,7 @@ namespace smt {
     }
 
     void theory_bv::unmerge_eh(theory_var v1, theory_var v2) {
+                
         // v1 was the root of the equivalence class
         // I must remove the zero_one_bits that are from v2.