code: func Sum(m []byte, key nacl.Key) *[Size]byte {
  	out := new([Size]byte)
  	poly1305.Sum(out, m, key)
  	return out
  }
docs: // Sum generates an authenticator for m using a one-time key and puts the result
  // (of length Size) into out.
  // Authenticating two different messages with the same key allows an attacker to
  // forge messages at will.