code: func MustSet(key string, value string) error {
  	gil.Lock()
  	defer gil.Unlock()
  	err := os.Setenv(key, value)
  	if err != nil {
  		return err
  	}
  	env[key] = value
  	return nil
  }
docs: // MustSet the value into the underlying ENV, as well as envy.
  // This may return an error if there is a problem setting the
  // underlying ENV value.