Clean up ItemSet#close!

lib/propane/parser/item_set.rb

@@ -110,17 +110,15 @@ class Propane
       #
       # @return [void]
       def close!
-        eval_items = @items
+        eval_items = @items.dup
         while eval_items.size > 0
-          this_eval_items = eval_items
-          eval_items = Set.new
-          this_eval_items.each do |item|
+          item = eval_items.first
+          eval_items.delete(item)
           item.closed_items.each do |new_item|
             unless @items.include?(new_item)
               eval_items << new_item
             end
           end
-          end
           @items += eval_items
         end
       end