corrected method enter and return and created those missing constants

lib/virtual/message.rb

@@ -20,6 +20,10 @@ module Virtual
   # During compilation Message and frame objects are created to do type analysis
 
   class Message
+    RETURN = 2
+    EXCEPTION = 3
+    SELF = 4
+
     def initialize me , normal , exceptional
       @me = me
       @next_normal = normal