code: func (k PrecomputedSharedKey) Unbox(nonce saltpack.Nonce, msg []byte) ([]byte, error) {
  	ret, ok := box.OpenAfterPrecomputation([]byte{}, msg, (*[24]byte)(&nonce), (*[32]byte)(&k))
  	if !ok {
  		return nil, saltpack.ErrDecryptionFailed
  	}
  	return ret, nil
  }
docs: // Unbox runs the unbox computation given a precomputed key.