add to_s for statements

lib/vool/variables.rb

@@ -18,6 +18,9 @@ module Vool
       end
       Mom::SlotDefinition.new(:message , [type , @name])
     end
+    def to_s
+      name.to_s
+    end
   end
 
   class InstanceVariable < Expression
@@ -29,6 +32,9 @@ module Vool
     def add_ivar( array )
       array << @name
     end
+    def to_s
+      "@#{name}"
+    end
   end
 
   class ClassVariable < Expression