derive Variable expression from name expression (to avoid the implementation that was there)

lib/ast/basic_expressions.rb

@@ -50,6 +50,9 @@ module Ast
     end
   end
 
+  class VariableExpression < NameExpression
+  end
+
   class ModuleName < NameExpression
   end
 

lib/ast/call_site_expression.rb

@@ -21,16 +21,4 @@ module Ast
       [:name , :args , :receiver]
     end
   end  
-  
-  class VariableExpression < CallSiteExpression
-
-    def initialize name
-      super( :_get_instance_variable  , [StringExpression.new(name)] )
-    end
-    def inspect
-      self.class.name + ".new(" + args[0].string.inspect + ")"  
-    end
-
-  end
-  
 end