class Rspec::Core::Configuration

def formatter=(formatter_to_use)
  formatter_class = case formatter_to_use.to_s
  when /doc/, 's', 'n'
    Rspec::Core::Formatters::DocumentationFormatter
  when 'progress' 
    Rspec::Core::Formatters::ProgressFormatter
  else 
    raise ArgumentError, "Formatter '#{formatter_to_use}' unknown - maybe you meant 'documentation' or 'progress'?."
  end
  options[:formatter_class] = formatter_class
end