# File lib/rspec/core/configuration.rb, line 572
      def add_formatter(formatter_to_use, *paths)
        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'?.")

        paths << output if paths.empty?
        formatters << formatter_class.new(*paths.map {|p| String === p ? file_at(p) : p})
      end