    # File lib/merb-core/controller/mixins/responder.rb, line 299
        def _perform_content_negotiation
          if fmt = params[:format] and !fmt.blank?
            accepts = [fmt.to_sym]
          else
            accepts = _accept_types
          end

          provided_formats = _provided_formats

          specifics = accepts & provided_formats
          return specifics.first unless specifics.length == 0
          return provided_formats.first if accepts.include?(:all) && !provided_formats.empty?

          message  = "A format (%s) that isn't provided (%s) has been requested. "
          message += "Make sure the action provides the format, and be "
          message += "careful of before filters which won't recognize "
          message += "formats provided within actions."
          raise Merb::ControllerExceptions::NotAcceptable,
            (message % [accepts.join(', '), provided_formats.join(', ')])
        end