Stop using dynamic_cast
diff --git a/expr.h b/expr.h
index 606ef90..e479ed8 100644
--- a/expr.h
+++ b/expr.h
@@ -41,6 +41,8 @@
 
   virtual Value* Compact() { return this; }
 
+  virtual bool IsLiteral() const { return false; }
+
   string DebugString() const;
 
  protected: