expose import model converter over Python, document it, add partial order axioms for lex, disable linear order axioms, prepare ground for re-adding clauses from reconstruction stack

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/api/python/z3/z3.py

@@ -6646,6 +6646,10 @@ class Solver(Z3PPObject):
         except Z3Exception:
             raise Z3Exception("model is not available")
 
+    def import_model_converter(self, other):
+        """Import model converter from other into the current solver"""
+        Z3_solver_import_model_converter(self.ctx.ref(), other.solver, self.solver)
+
     def unsat_core(self):
         """Return a subset (as an AST vector) of the assumptions provided to the last check().
 
@@ -8310,11 +8314,17 @@ def AtLeast(*args):
     _args, sz = _to_ast_array(args1)
     return BoolRef(Z3_mk_atleast(ctx.ref(), sz, _args, k), ctx)
 
+def _reorder_pb_arg(arg):
+    a, b = arg
+    if not _is_int(b) and _is_int(a):
+        return b, a
+    return arg
 
 def _pb_args_coeffs(args, default_ctx = None):
     args  = _get_args_ast_list(args)
     if len(args) == 0:
        return _get_ctx(default_ctx), 0, (Ast * 0)(), (ctypes.c_int * 0)()
+    args = [_reorder_pb_arg(arg) for arg in args]
     args, coeffs = zip(*args)
     if z3_debug():
         _z3_assert(len(args) > 0, "Non empty list of arguments expected")

src/api/z3_api.h

@@ -6213,6 +6213,14 @@ extern "C" {
 
     /**
        \brief Ad-hoc method for importing model conversion from solver.
+
+       This method is used for scenarios where \c src has been used to solve a set
+       of formulas and was interrupted. The \c dst solver may be a strengthening of \c src
+       obtained from cubing (assigning a subset of literals or adding constraints over the 
+       assertions available in \c src). If \c dst ends up being satisfiable, the model for \c dst
+       may not correspond to a model of the original formula due to inprocessing in \c src.
+       This method is used to take the side-effect of inprocessing into account when returning
+       a model for \c dst.
        
        def_API('Z3_solver_import_model_converter', VOID, (_in(CONTEXT), _in(SOLVER), _in(SOLVER)))
      */

src/sat/sat_model_converter.cpp

@@ -23,16 +23,12 @@ Revision History:
 
 namespace sat {
 
-    model_converter::model_converter(): m_solver(nullptr) {
+    model_converter::model_converter(): m_solver(nullptr), m_exposed_lim(0) {
     }
 
     model_converter::~model_converter() {
-        reset();
     }
 
-    void model_converter::reset() {
-        m_entries.finalize();
-    }
 
     model_converter& model_converter::operator=(model_converter const& other) {
         copy(other);
@@ -40,21 +36,19 @@ namespace sat {
     }
 
     bool model_converter::legal_to_flip(bool_var v) const {
-        // std::cout << "check " << v << " " << m_solver << "\n";
         if (m_solver && m_solver->is_assumption(v)) {
-            std::cout << "flipping assumption v" << v << "\n";
+            IF_VERBOSE(0, verbose_stream() << "flipping assumption v" << v << "\n";);
             UNREACHABLE();
             throw solver_exception("flipping assumption");
         }
         if (m_solver && m_solver->is_external(v) && m_solver->is_incremental()) {
-            std::cout << "flipping external v" << v << "\n";
+            IF_VERBOSE(0, verbose_stream() << "flipping external v" << v << "\n";);
             UNREACHABLE();
             throw solver_exception("flipping external");
         }
         return !m_solver || !m_solver->is_assumption(v);
     }
 
-
     void model_converter::process_stack(model & m, literal_vector const& c, elim_stackv const& stack) const {
         SASSERT(!stack.empty());
         unsigned sz = stack.size();
@@ -73,46 +67,33 @@ 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 =  false; 
-        //SASSERT(!m_solver || m_solver->check_clauses(m));
+        literal_vector clause;
-        while (it != begin) {
+        for (unsigned i = m_entries.size(); i-- > m_exposed_lim; ) {
-            --it;
+            entry const& e = m_entries[i];
-            bool_var v0 = it->var();
+            bool_var v0 = e.var();
-            SASSERT(it->get_kind() != ELIM_VAR || v0 == null_bool_var || m[v0] == l_undef);
+            SASSERT(e.get_kind() != ELIM_VAR || v0 == null_bool_var || m[v0] == l_undef);
-            // if it->get_kind() == BCE, then it might be the case that m[v] != l_undef,
+            // if e.get_kind() == BCE, then it might be the case that m[v] != l_undef,
             // and the following procedure flips its value.
             bool sat = false;
             bool var_sign = false;
             unsigned index = 0;
-            literal_vector clause;
+            clause.reset();
             VERIFY(v0 == null_bool_var || legal_to_flip(v0));
-            for (literal l : it->m_clauses) {
+            for (literal l : e.m_clauses) {
                 if (l == null_literal) {
                     // end of clause
-                    if (!sat && it->get_kind() == ATE) {
+                    VERIFY (sat || e.get_kind() != ATE);
-                        IF_VERBOSE(0, display(verbose_stream() << "violated ate\n", *it) << "\n");
+                    if (!sat && e.get_kind() != ATE && v0 != null_bool_var) {     
-                        IF_VERBOSE(0, for (unsigned v = 0; v < m.size(); ++v) verbose_stream() << v << " := " << m[v] << "\n";);
-                        IF_VERBOSE(0, display(verbose_stream()));
-                        UNREACHABLE();
-                        first = false;
-                    }
-                    if (!sat && it->get_kind() != ATE && v0 != null_bool_var) {     
                         VERIFY(legal_to_flip(v0));                        
                         m[v0] = var_sign ? l_false : l_true;
-                        //IF_VERBOSE(0, verbose_stream() << "assign " << v0 << " "<< m[v0] << "\n");
                     }
-                    elim_stack* st = it->m_elim_stack[index];
+                    elim_stack* st = e.m_elim_stack[index];
                     if (st) {
                         process_stack(m, clause, st->stack());
                     }
                     sat = false;
-                    if (first && m_solver && !m_solver->check_clauses(m)) {
+                    VERIFY(!first || !m_solver || m_solver->check_clauses(m));
-                        IF_VERBOSE(0, display(verbose_stream() << "after processing stack\n", *it) << "\n");
-                        IF_VERBOSE(0, display(verbose_stream()));
-                        first = false;
-                    }
                     ++index;
                     clause.reset();
                     continue;                    
@@ -123,7 +104,6 @@ namespace sat {
                     continue;
                 bool sign  = l.sign();
                 bool_var v = l.var();
-                if (v >= m.size()) std::cout << v << " model size: " << m.size() << "\n";
                 VERIFY(v < m.size());
                 if (v == v0)
                     var_sign = sign;
@@ -133,25 +113,21 @@ namespace sat {
                     VERIFY(legal_to_flip(v));
                     // clause can be satisfied by assigning v.
                     m[v] = sign ? l_false : l_true;
-                    // if (first) std::cout << "set: " << l << "\n";
                     sat = true;
-                    if (first && m_solver && !m_solver->check_clauses(m)) {
+                    VERIFY(!first || !m_solver || m_solver->check_clauses(m));
-                        IF_VERBOSE(0, display(verbose_stream() << "after flipping undef\n", *it)  << "\n");
-                        first = false;
-                    }
                 }
             }
             DEBUG_CODE({
                 // all clauses must be satisfied
                 bool sat = false;
                 bool undef = false;
-                for (literal const& l : it->m_clauses) {
+                for (literal const& l : e.m_clauses) {
                     if (l == null_literal) {
                         CTRACE("sat", !sat, 
                                if (m_solver) m_solver->display(tout);
                                display(tout);
                                for (unsigned v = 0; v < m.size(); ++v) tout << v << ": " << m[v] << "\n";
-                               for (literal const& l2 : it->m_clauses) {
+                               for (literal const& l2 : e.m_clauses) {
                                    if (l2 == null_literal) tout << "\n"; else tout << l2 << " ";
                                    if (&l == &l2) break;
                                }
@@ -179,24 +155,21 @@ namespace sat {
     */
     bool model_converter::check_model(model const & m) const {
         bool ok = true;
-        vector<entry>::const_iterator begin = m_entries.begin();
+        for (entry const & e : m_entries) {
-        vector<entry>::const_iterator it    = m_entries.end();
-        while (it != begin) {
-            --it;
             bool sat = false;
-            literal_vector::const_iterator it2     = it->m_clauses.begin();
+            literal_vector::const_iterator it      = e.m_clauses.begin();
-            literal_vector::const_iterator itbegin = it2;
+            literal_vector::const_iterator itbegin = it;
-            literal_vector::const_iterator end2    = it->m_clauses.end();
+            literal_vector::const_iterator end     = e.m_clauses.end();
-            for (; it2 != end2; ++it2) {
+            for (; it != end; ++it) {
-                literal l  = *it2;
+                literal l  = *it;
                 if (l == null_literal) {
                     // end of clause
                     if (!sat) {
-                        TRACE("sat_model_bug", tout << "failed eliminated: " << mk_lits_pp(static_cast<unsigned>(it2 - itbegin), itbegin) << "\n";);
+                        TRACE("sat_model_bug", tout << "failed eliminated: " << mk_lits_pp(static_cast<unsigned>(it - itbegin), itbegin) << "\n";);
                         ok = false;
                     }
                     sat = false;
-                    itbegin = it2;
+                    itbegin = it;
                     itbegin++;
                     continue;
                 }
@@ -239,6 +212,16 @@ namespace sat {
         stackv().reset();
     }
 
+    void model_converter::set_clause(entry & e, literal l1, literal l2) {
+        e.m_clause.push_back(l1);
+        e.m_clause.push_back(l2);
+    }
+
+    void model_converter::set_clause(entry & e, clause const & c) {
+        e.m_clause.append(c.size(), c.begin());
+    }
+
+
     void model_converter::insert(entry & e, clause const & c) {
         SASSERT(c.contains(e.var()));
         SASSERT(m_entries.begin() <= &e);
@@ -349,27 +332,27 @@ namespace sat {
             out << l;
         }
         out << ")";
-        for (literal l : entry.m_clauses) {
-            if (l != null_literal && l.var() != null_bool_var) {
-                if (false && m_solver && m_solver->was_eliminated(l.var())) out << "\neliminated: " << l;
-            }
-        }
         return out;
     }
 
     void model_converter::copy(model_converter const & src) {
-        reset();
+        m_entries.reset();        
         m_entries.append(src.m_entries);
+        m_exposed_lim = src.m_exposed_lim;
     }
 
     void model_converter::flush(model_converter & src) {
         VERIFY(this != &src);
         m_entries.append(src.m_entries);
+        m_exposed_lim = src.m_exposed_lim;
         src.m_entries.reset();
+        src.m_exposed_lim = 0;
     }
 
     void model_converter::collect_vars(bool_var_set & s) const {
-        for (entry const & e : m_entries) s.insert(e.m_var);
+        for (entry const & e : m_entries) {
+            s.insert(e.m_var);
+        }
     }
 
     unsigned model_converter::max_var(unsigned min) const {
@@ -398,7 +381,8 @@ namespace sat {
 
     void model_converter::expand(literal_vector& update_stack) {
         sat::literal_vector clause;
-        for (entry const& e : m_entries) {
+        for (unsigned i = m_exposed_lim; i < m_entries.size(); ++i) {
+            entry const& e = m_entries[i];
             unsigned index = 0;
             clause.reset();
             for (literal l : e.m_clauses) {
@@ -427,6 +411,56 @@ namespace sat {
                 }
             }
         }
+        m_exposed_lim = m_entries.size();
+    }
+
+    void model_converter::init_search(solver& s) {
+#if 0
+        unsigned j = 0, k = 0;
+        literal_vector clause;
+        for (unsigned i = 0; i < m_entries.size(); ++i) {
+            entry & e = m_entries[i];
+            if (!m_mark[e.var()]) {
+                m_entries[j++] = e;
+                if (i < m_exposed_lim) k++;
+                continue;
+            }
+            clause.reset();
+            // For covered clauses we record the original clause. The role of m_clauses is to record ALA
+            // tautologies and are not part of the clause that is removed.
+
+            if (!e.m_clause.empty()) {
+                clause.append(e.m_clause);
+                s.mk_clause(clause.size(), clause.c_ptr(), false);
+                continue;
+            }
+            for (literal lit : e.m_clauses) {
+                if (lit == null_literal) {
+                    s.mk_clause(clause.size(), clause.c_ptr(), false);
+                    clause.reset();
+                }
+                else {
+                    clause.push_back(lit);
+                }
+            }
+        }
+        m_entries.shrink(j);
+        m_exposed_lim = k;
+        for (bool& m : m_mark) {
+            m = false;
+        }
+#endif
+    }
+
+    void model_converter::add_clause(unsigned n, literal const* lits) {
+        if (m_entries.empty()) {
+            return;
+        }
+        // TBD: we just mark variables instead of literals because entries don't have directly literal information.
+        for (unsigned i = 0; i < n; ++i) {
+            m_mark.reserve(lits[i].var() + 1);
+            m_mark[lits[i].var()] = true;
+        }
     }
 
 };

src/sat/sat_model_converter.h

@@ -72,20 +72,25 @@ namespace sat {
             bool_var                m_var;
             kind                    m_kind;
             literal_vector          m_clauses; // the different clauses are separated by null_literal
+            literal_vector          m_clause;  // original clause in case of CCE
             sref_vector<elim_stack> m_elim_stack;
             entry(kind k, bool_var v): m_var(v), m_kind(k) {}
         public:
             entry(entry const & src):
                 m_var(src.m_var),
                 m_kind(src.m_kind),
-                m_clauses(src.m_clauses) {
+                m_clauses(src.m_clauses),
+                m_clause(src.m_clause)
+            {
                 m_elim_stack.append(src.m_elim_stack);
             }
             bool_var var() const { return m_var; }
             kind get_kind() const { return m_kind; }
         };
     private:
-        vector<entry>          m_entries;
+        vector<entry>          m_entries;           // entries accumulated during SAT search
+        unsigned               m_exposed_lim;       // last entry that was exposed to model converter.
+        svector<bool>          m_mark;              // literals that are used in asserted clauses.
         solver const*          m_solver;
         elim_stackv            m_elim_stack;
 
@@ -113,6 +118,8 @@ namespace sat {
         void insert(entry & e, literal l1, literal l2);
         void insert(entry & e, clause_wrapper const & c);
         void insert(entry & c, literal_vector const& covered_clause);
+        void set_clause(entry & e, literal l1, literal l2);
+        void set_clause(entry & e, clause const & c);
 
         void add_ate(literal_vector const& lits);
         void add_ate(literal l1, literal l2);
@@ -121,7 +128,9 @@ namespace sat {
         bool empty() const { return m_entries.empty(); }
         unsigned size() const { return m_entries.size(); }
 
-        void reset();
+        void init_search(solver& s);
+        void add_clause(unsigned n, literal const* lits);
+
         bool check_invariant(unsigned num_vars) const;
         void display(std::ostream & out) const;
         bool check_model(model const & m) const;

src/sat/sat_simplifier.cpp

@@ -1223,8 +1223,6 @@ namespace sat {
             }
             for (unsigned i = idx; i > 0; --i) {
                 literal lit = m_covered_clause[i];
-                //s.mark_visited(lit);
-                //continue;
                 if (!s.is_marked(lit)) continue;
                 clause_ante const& ante = m_covered_antecedent[i];
                 if (ante.cls()) {
@@ -1591,6 +1589,7 @@ namespace sat {
                 }
             }
             m_mc.insert(new_entry, m_covered_clause);
+            m_mc.set_clause(new_entry, c);
         }
         
         void block_covered_binary(watched const& w, literal l1, literal blocked, model_converter::kind k) {
@@ -1600,6 +1599,7 @@ namespace sat {
             TRACE("blocked_clause", tout << "new blocked clause: " << l2 << " " << l1 << "\n";);
             s.set_learned(l1, l2);
             m_mc.insert(new_entry, m_covered_clause);
+            m_mc.set_clause(new_entry, l1, l2);
             m_queue.decreased(~l2);
         }
 

src/sat/sat_solver.cpp

@@ -357,6 +357,9 @@ namespace sat {
                 m_drat.add(m_lemma);
             }
             ++m_stats.m_non_learned_generation;
+            if (!m_searching) {
+                m_mc.add_clause(num_lits, lits);
+            }
         }
         
         switch (num_lits) {
@@ -1832,8 +1835,8 @@ namespace sat {
         m_min_core_valid = false;
         m_min_core.reset();
         m_simplifier.init_search();
+        m_mc.init_search(*this);
         TRACE("sat", display(tout););
-
     }
 
     bool solver::should_simplify() const {
@@ -4485,13 +4488,14 @@ namespace sat {
     lbool solver::get_bounded_consequences(literal_vector const& asms, bool_var_vector const& vars, vector<literal_vector>& conseq) {
         bool_var_set unfixed_vars;
         unsigned num_units = 0, num_iterations = 0;
-        for (unsigned i = 0; i < vars.size(); ++i) {
+        for (bool_var v : vars) {
-            unfixed_vars.insert(vars[i]);
+            unfixed_vars.insert(v);
         }
         TRACE("sat", tout << asms << "\n";);
         m_antecedents.reset();
         pop_to_base_level();
         if (inconsistent()) return l_false;
+        flet<bool> _searching(m_searching, true);
         init_search();
         propagate(false);
         if (inconsistent()) return l_false;

src/sat/sat_solver.h

@@ -348,7 +348,6 @@ namespace sat {
 
         bool limit_reached() {
             if (!m_rlimit.inc()) {
-                m_mc.reset();
                 m_model_is_current = false;
                 TRACE("sat", tout << "canceled\n";);
                 m_reason_unknown = "sat.canceled";

src/sat/tactic/goal2sat.cpp

@@ -934,7 +934,6 @@ void sat2goal::mc::flush_smc(sat::solver_core& s, atom2bool_var const& map) {
 void sat2goal::mc::flush_gmc() {
     sat::literal_vector updates;
     m_smc.expand(updates);    
-    m_smc.reset();
     if (!m_gmc) m_gmc = alloc(generic_model_converter, m, "sat2goal");
     // now gmc owns the model converter
     sat::literal_vector clause;

src/smt/smt_theory.cpp

@@ -120,6 +120,9 @@ namespace smt {
     }
 
     literal theory::mk_eq(expr * a, expr * b, bool gate_ctx) {
+        if (a == b) {
+            return true_literal;
+        }
         context & ctx = get_context();
         app * eq = ctx.mk_eq_atom(a, b);
         TRACE("mk_var_bug", tout << "mk_eq: " << eq->get_id() << " " << a->get_id() << " " << b->get_id() << "\n";

src/smt/theory_seq.cpp

@@ -268,6 +268,7 @@ theory_seq::theory_seq(ast_manager& m, theory_seq_params const & params):
     m_params(params),
     m_rep(m, m_dm),
     m_reset_cache(false),
+    m_lts_checked(false),
     m_eq_id(0),
     m_find(*this),
     m_overlap(m),
@@ -336,12 +337,16 @@ final_check_status theory_seq::final_check_eh() {
         ++m_stats.m_solve_eqs;
         TRACEFIN("solve_eqs");
         return FC_CONTINUE;
-    }
+    }    
     if (check_contains()) {
         ++m_stats.m_propagate_contains;
         TRACEFIN("propagate_contains");
         return FC_CONTINUE;
     }
+    if (check_lts()) {
+        TRACEFIN("check_lts");
+        return FC_CONTINUE;
+    }
     if (solve_nqs(0)) {
         ++m_stats.m_solve_nqs;
         TRACEFIN("solve_nqs");
@@ -2147,6 +2152,57 @@ bool theory_seq::check_contains() {
     }
     return m_new_propagation || ctx.inconsistent();
 }
+
+bool theory_seq::check_lts() {
+    context& ctx = get_context();
+    if (m_lts.empty() || m_lts_checked) {
+        return false;
+    }
+    unsigned sz = m_lts.size();
+    m_trail_stack.push(value_trail<theory_seq, bool>(m_lts_checked));
+    m_lts_checked = true;
+    expr* a = nullptr, *b = nullptr, *c = nullptr, *d = nullptr;
+    bool is_strict1, is_strict2;
+    for (unsigned i = 0; i + 1 < sz; ++i) {
+        expr* p1 = m_lts[i];
+        VERIFY(m_util.str.is_lt(p1, a, b) || m_util.str.is_le(p1, a, b));
+        literal r1 = ctx.get_literal(p1);
+        if (ctx.get_assignment(r1) == l_false) {
+            std::swap(a, b);
+            r1.neg();
+            is_strict1 = m_util.str.is_le(p1);
+        }
+        else {
+            is_strict1 = m_util.str.is_lt(p1);
+        }
+        for (unsigned j = i + 1; j < sz; ++j) {
+            expr* p2 = m_lts[j];
+            VERIFY(m_util.str.is_lt(p2, c, d) || m_util.str.is_le(p2, c, d));
+            literal r2 = ctx.get_literal(p2);
+            if (ctx.get_assignment(r2) == l_false) {
+                std::swap(c, d);
+                r2.neg();
+                is_strict2 = m_util.str.is_le(p2);
+            }
+            else {
+                is_strict2 = m_util.str.is_lt(p2);
+            }
+            if (ctx.get_enode(b)->get_root() == ctx.get_enode(c)->get_root()) {
+
+                literal eq = (b == c) ? true_literal : mk_eq(b, c, false);
+                bool is_strict = is_strict1 || is_strict2; 
+                if (is_strict) {
+                    add_axiom(~r1, ~r2, ~eq, mk_literal(m_util.str.mk_lex_lt(a, d)));
+                }
+                else {
+                    add_axiom(~r1, ~r2, ~eq, mk_literal(m_util.str.mk_lex_le(a, d)));
+                }
+            }
+        }
+    }
+    return true;
+}
+
 /*
    - Eqs = 0
    - Diseqs evaluate to false
@@ -5491,7 +5547,7 @@ void theory_seq::assign_eh(bool_var v, bool is_true) {
         // no-op
     }
     else if (m_util.str.is_lt(e) || m_util.str.is_le(e)) {
-        // no-op
+        m_lts.push_back(e);
     }
     else {
         TRACE("seq", tout << mk_pp(e, m) << "\n";);
@@ -5619,6 +5675,7 @@ void theory_seq::push_scope_eh() {
     m_eqs.push_scope();
     m_nqs.push_scope();
     m_ncs.push_scope();
+    m_lts.push_scope();
     m_internal_nth_lim.push_back(m_internal_nth_es.size());
 }
 
@@ -5632,6 +5689,7 @@ void theory_seq::pop_scope_eh(unsigned num_scopes) {
     m_eqs.pop_scope(num_scopes);
     m_nqs.pop_scope(num_scopes);
     m_ncs.pop_scope(num_scopes);
+    m_lts.pop_scope(num_scopes);
     m_rewrite.reset();    
     if (ctx.get_base_level() > ctx.get_scope_level() - num_scopes) {
         m_replay.reset();
@@ -5927,10 +5985,10 @@ void theory_seq::propagate_not_suffix(expr* e) {
    !(e1 < e2) => e1 = e2 or e2 = empty or e2 = xdz
    !(e1 < e2) => e1 = e2 or e2 = empty or d < c
    !(e1 < e2) => e1 = e2 or e1 = xcy
+   !(e1 = e2) or !(e1 < e2)
    
 optional:
    e1 < e2 or e1 = e2 or e2 < e1
-   !(e1 = e2) or !(e1 < e2)
    !(e1 = e2) or !(e2 < e1)
    !(e1 < e2) or !(e2 < e1)
  */
@@ -5962,13 +6020,7 @@ void theory_seq::add_lt_axiom(expr* n) {
     add_axiom(lt, eq, e1xcy);
     add_axiom(lt, eq, emp2, ltdc);
     add_axiom(lt, eq, emp2, e2xdz);
-    if (e1->get_id() <= e2->get_id()) {
+    add_axiom(~eq, ~lt);
-        literal gt = mk_literal(m_util.str.mk_lex_lt(e2, e1));
-        add_axiom(lt, eq, gt);
-        add_axiom(~eq, ~lt);
-        add_axiom(~eq, ~gt);
-        add_axiom(~lt, ~gt);
-    }
 }
 
 /**

src/smt/theory_seq.h

@@ -328,6 +328,8 @@ namespace smt {
         scoped_vector<eq>          m_eqs;        // set of current equations.
         scoped_vector<ne>          m_nqs;        // set of current disequalities.
         scoped_vector<nc>          m_ncs;        // set of non-contains constraints.
+        scoped_vector<expr*>       m_lts;        // set of asserted str.<, str.<= literals
+        bool                       m_lts_checked; 
         unsigned                   m_eq_id;
         th_union_find              m_find;
 
@@ -460,6 +462,7 @@ namespace smt {
 
         bool check_extensionality();
         bool check_contains();
+        bool check_lts();
         bool solve_eqs(unsigned start);
         bool solve_eq(expr_ref_vector const& l, expr_ref_vector const& r, dependency* dep, unsigned idx);
         bool simplify_eq(expr_ref_vector& l, expr_ref_vector& r, dependency* dep);