pulled assignment as special case from the operators

lib/ast/operator_expressions.rb

@@ -16,4 +16,22 @@ module Ast
       "#{left} #{operator} #{right}"
     end
   end
-end
+
+  class AssignmentExpression < Expression
+    attr_reader  :left, :right
+
+    def initialize left, right
+      @left, @right = left, right
+    end
+    def attributes
+      [:left, :right]
+    end
+    def inspect
+      self.class.name + ".new(" +  left.inspect + "," + right.inspect + ")"  
+    end
+    def to_s
+      "#{left} = #{right}"
+    end
+  end
+
+end