You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: router.rb
    path of file: ./repos/rack-app/lib/rack/app
    
    the code of the file until where you have to start completion: class Rack::App::Router
  require 'rack/app/router/tree'
  require 'rack/app/router/error'

  attr_reader :tree

  NOT_FOUND_APP = lambda do |_env|
    rack_response = Rack::Response.new
    rack_response.status = 404
    rack_response.write('404 Not Found')
    rack_response.finish
  end

  def call(env)
    env[Rack::App::Constants::ENV::ROUTER] = self
    @tree.call
    
  def merge_router!(router, prop = {})
    raise(ArgumentError, 'invalid router object, must implement :endpoints interface') unless router.respond_to?(:endpoints)
    router.endpoints.each do |endpoint|
      new_request_path = ::Rack::App::Utils.join(prop[:namespaces], endpoint.request_path)
      new_ancestors = endpoint.config.ancestor_apps + [prop[:new_ancestor]]
      new_endpoint = endpoint.fork(:request_path => new_request_path, :ancestors => new_ancestors)
      register_endpoint!(new_endpoint)
    end
    nil
  end