add API for setting variable activity

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

src/solver/combined_solver.cpp

@@ -328,6 +328,11 @@ public:
             return m_solver2->get_trail();
     }
 
+    void set_activity(expr* lit, double activity) override {
+        m_solver1->set_activity(lit, activity);
+        m_solver2->set_activity(lit, activity);
+    }
+
     proof * get_proof() override {
         if (m_use_solver1_results)
             return m_solver1->get_proof();

src/solver/solver.h

@@ -251,6 +251,8 @@ public:
     
     virtual void get_levels(ptr_vector<expr> const& vars, unsigned_vector& depth) = 0;
 
+    virtual void set_activity(expr* lit, double activity) = 0;
+
     class scoped_push {
         solver& s;
         bool    m_nopop;

src/solver/solver_pool.cpp

@@ -127,6 +127,10 @@ public:
         return m_base->get_trail();
     }
 
+    void set_activity(expr* var, double activity) override {
+        m_base->set_activity(var, activity);
+    }
+
     lbool check_sat_core2(unsigned num_assumptions, expr * const * assumptions) override {
         SASSERT(!m_pushed || get_scope_level() > 0);
         m_proof.reset();

src/solver/tactic2solver.cpp

@@ -93,6 +93,11 @@ public:
         throw default_exception("cannot retrieve trail from solvers created using tactcis");
     }
 
+    void set_activity(expr* var, double activity) override {
+        throw default_exception("cannot set activity for solvers created using tactcis");
+    }
+
+
 };
 
 ast_manager& tactic2solver::get_manager() const { return m_assertions.get_manager(); }