class Puma::Rack::Builder

def initialize(default_app = nil, &block)
  @use, @map, @run, @warmup = [], nil, default_app, nil
  # Conditionally load rack now, so that any rack middlewares,
  # etc are available.
  begin
    require 'rack'
  rescue LoadError
  end
  instance_eval(&block) if block
end