# File lib/rspec/core/configuration.rb, line 220
      def formatter=(formatter_to_use)
        self.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'?.")
      end