code: func (k *Key) Container() *Key {
  	if len(k.pairs) == 2 {
  		return nil
  	}
  	cont, err := k.set.Key(k.pairs[:len(k.pairs)-2]...)
  	if err != nil {
  		panic(err) // 'k' has been validated, all its components are thus also valid
  	}
  	return cont
  }
docs: // Container returns a key with all (kind, id) pairs of this key, except the
  // last one, or nil if this key has only one (kind, id) pair.
  //
  // Usually it represents a container that holds an object represented by the
  // this key.