ohm-snippet.rb

      def on(error, message = (block_given? ? yield : raise(ArgumentError)))
        handle(error) do
          @output << message
        end
      end

    protected

      def handle(error)
        if (errors = @errors.select {|e| error === e }).any?
          @unhandled -= errors
          yield(errors) if block_given?
        end
      end