code: func Verify(digest *[Size]byte, m []byte, key nacl.Key) bool {
  	mac := hmac.New(sha512.New, (*key)[:])
  	mac.Write(m)
  	expectedMAC := mac.Sum(nil) // first 256 bits of 512 bit sum
  	return hmac.Equal((*digest)[:], expectedMAC[:Size])
  }
docs: // Verify checks that digest is a correct authenticator of a message m under the
  // secret key key. If not, the function returns false. Verify does not leak
  // timing information.