class Rack::Cascade

def add(app)

be tried last.
Append an app to the list of apps to cascade. This app will 
def add(app)
  @apps << app
end

def call(env)

return the response from the last app.
cascading, try the next app. If all responses require cascading,
Call each app in order. If the responses uses a status that requires 
def call(env)
  return [404, { CONTENT_TYPE => "text/plain" }, []] if @apps.empty?
  result = nil
  last_body = nil
  @apps.each do |app|
    # The SPEC says that the body must be closed after it has been iterated
    # by the server, or if it is replaced by a middleware action. Cascade
    # replaces the body each time a cascade happens. It is assumed that nil
    # does not respond to close, otherwise the previous application body
    # will be closed. The final application body will not be closed, as it
    # will be passed to the server as a result.
    last_body.close if last_body.respond_to? :close
    result = app.call(env)
    return result unless @cascade_for.include?(result[0].to_i)
    last_body = result[2]
  end
  result
end

def include?(app)

Whether the given app is one of the apps to cascade to. 
def include?(app)
  @apps.include?(app)
end

def initialize(apps, cascade_for = [404, 405])

from an app, the next app is tried.
cascade_for: The statuses to use cascading for. If a response is received
apps: An enumerable of rack applications.

cascading. Arguments:
Set the apps to send requests to, and what statuses result in 
def initialize(apps, cascade_for = [404, 405])
  @apps = []
  apps.each { |app| add app }
  @cascade_for = {}
  [*cascade_for].each { |status| @cascade_for[status] = true }
end