diff --git a/include/xo/interpreter2/VirtualSchematikaMachine.hpp b/include/xo/interpreter2/VirtualSchematikaMachine.hpp
index 774659eb..d06deb26 100644
--- a/include/xo/interpreter2/VirtualSchematikaMachine.hpp
+++ b/include/xo/interpreter2/VirtualSchematikaMachine.hpp
@@ -120,12 +120,18 @@ namespace xo {
              **/
             void _do_eval_lambda_op();
 
-            /** evaluate a variable expression
+            /** evaluate variable expression (definition)
              *  Require:
              *  - expression in @ref expr_
              **/
             void _do_eval_variable_op();
 
+            /** evaluate a variable reference (use after definition)
+             *  Require:
+             *  - expression in @ref expr_
+             **/
+            void _do_eval_varref_op();
+
             /** evaluate an apply expression
              *  Require:
              * - expression in @ref expr_
diff --git a/src/interpreter2/VirtualSchematikaMachine.cpp b/src/interpreter2/VirtualSchematikaMachine.cpp
index b32f6b24..1395209a 100644
--- a/src/interpreter2/VirtualSchematikaMachine.cpp
+++ b/src/interpreter2/VirtualSchematikaMachine.cpp
@@ -167,6 +167,9 @@ namespace xo {
             case exprtype::variable:
                 _do_eval_variable_op();
                 break;
+            case exprtype::varref:
+                _do_eval_varref_op();
+                break;
             case exprtype::apply:
                 _do_eval_apply_op();
                 break;
@@ -210,6 +213,13 @@ namespace xo {
             assert(false);
         }
 
+        void
+        VirtualSchematikaMachine::_do_eval_varref_op()
+        {
+            // not implemented
+            assert(false);
+        }
+
         void
         VirtualSchematikaMachine::_do_eval_apply_op()
         {