def -(right)
      result = CatOnYaml.new
      @entries.each do |e|
        result.add_entry(e) unless right.contains(e)
      end
      result
    end
def -(right)
      result = CatOnYaml.new
      @entries.each do |e|
        result.add_entry(e) unless right.contains(e)
      end
      result
    end
return differences from this catalog and right catalog
 param [Cat] right
 @return [CatOnYaml]