class Rack::Builder

is given, it is evaluated in the context of the instance.
default application if +run+ is not called later. If a block
Initialize a new Rack::Builder instance. +default_app+ specifies the

def initialize(default_app = nil, &block)
  @use = []
  @map = nil
  @run = default_app
  @warmup = nil
  @freeze_app = false
  instance_eval(&block) if block_given?
end