class RSpec::Core::Configuration

def formatter=(formatter_to_use)
  if formatter_to_use.is_a?(Class)
    formatter_class = formatter_to_use
  else
    formatter_class = case formatter_to_use.to_s
    when 'd', 'doc', 'documentation', 's', 'n', 'spec', 'nested'
      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
  end
  self.formatter_class = formatter_class
end