```ruby
require 'rubygems'
require 'rack'

$LOAD_PATH.unshift APP_ROOT

$app = eval "Rack::Builder.new { #{File.read APP_ROOT + '/config.ru'} }"```

Load a version of Rack denoted by RACK_VERSION
user
```ruby
require 'rubygems'

gem 'rack', RACK_VERSION
require 'rack'

$LOAD_PATH.unshift APP_ROOT

$app = eval "Rack::Builder.new { #{File.read APP_ROOT + '/config.ru'} }"```
assistant