/**
 * Ensure a value or object will remain globally unique
 * @param  string  $key   The value or object name
 * @param  Closure        The closure that defines the object
 * @return mixed
 */
public function singleton($key, $value)
{
    $this->set($key, function ($c) use ($value) {
        static $object;

        if (null === $object) {
            $object = $value($c);
        }

        return $object;
    });
}