code: public function get($key)
      {
          if (!array_key_exists($key, static::$map)) {
              throw new NotFoundException($key);
          }
          if (static::$map[$key] instanceof ReflectionFunction) {
              static::$map[$key] = static::$map[$key]->invoke($this);
          }
          return static::$map[$key];
      }
docs: Resolve the dependency identified by `$key`. Note `$key` is not type
  hinted as it would violate the `Psr\Container\ContainerInterface`.
  
  @param string $key The unique identifier for the dependency.
  @return mixed Whatever was stored under $key.
  @throws Disclosure\NotFoundException if no such $key was registered.