class RSpec::Core::Configuration

def add_formatter(formatter_to_use, path=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(path ? file_at(path) : output)
end