fix #2468, adding assignment phase heuristic

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

src/smt/smt_theory.h

@@ -143,6 +143,13 @@ namespace smt {
         virtual void assign_eh(bool_var v, bool is_true) {
         }
 
+        /**
+           \brief use the theory to determine phase of the variable.
+         */
+        virtual lbool get_phase(bool_var v) {
+            return l_undef;
+        }
+
         /**
            \brief Equality propagation (v1 = v2): Core -> Theory
         */