# File lib/rspec/core/configuration.rb, line 267
      def add_formatter(formatter_to_use, output=nil)
        formatter_class =
          built_in_formatter(formatter_to_use) ||
          custom_formatter(formatter_to_use) ||
          (raise ArgumentError, "Formatter '#{formatter_to_use}' unknown - maybe you meant 'documentation' or 'progress'?.")

        formatters << formatter_class.new(output ? File.new(output, 'w') : self.output)
      end