adding trail/levels

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

src/smt/smt_kernel.h

@@ -219,6 +219,16 @@ namespace smt {
         */
         expr* next_decision();
 
+        /**
+           \brief retrieve depth of variables from decision stack.
+        */
+        void get_levels(ptr_vector<expr> const& vars, unsigned_vector& depth);
+
+        /**
+           \brief retrieve trail of assignment stack.
+        */
+        expr_ref_vector get_trail();
+
         /**
            \brief (For debubbing purposes) Prints the state of the kernel
         */