add facility to add lemmas

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

src/solver/combined_solver.cpp

@@ -179,6 +179,12 @@ public:
         m_solver2->assert_expr(t, a);
     }
 
+    virtual void assert_lemma(expr* t) {
+        m_solver1->assert_lemma(t);
+        if (m_solver2_initialized)
+            m_solver2->assert_lemma(t);
+    }
+
     virtual void push() {
         switch_inc_mode();
         m_solver1->push();

src/solver/solver.h

@@ -90,6 +90,12 @@ public:
     */
     virtual void assert_expr(expr * t, expr * a) = 0;
 
+    /**
+       \brief Add a lemma to the assertion stack. A lemma is assumed to be a consequence of already
+       asserted formulas. The solver is free to ignore lemmas.
+    */
+    virtual void assert_lemma(expr * t) = 0;
+
     /**
        \brief Create a backtracking point.
     */

src/solver/tactic2solver.cpp

@@ -55,6 +55,7 @@ public:
     virtual void set_produce_models(bool f) { m_produce_models = f; }
 
     virtual void assert_expr(expr * t);
+    virtual void assert_lemma(expr * t);
 
     virtual void push_core();
     virtual void pop_core(unsigned n);
@@ -115,6 +116,9 @@ void tactic2solver::assert_expr(expr * t) {
     m_result = 0;
 }
 
+void tactic2solver::assert_lemma(expr * t) {    
+}
+
 void tactic2solver::push_core() {
     m_scopes.push_back(m_assertions.size());
     m_result = 0;