code: func Precompute(peersPublicKey, privateKey nacl.Key) nacl.Key {
  	sharedKey := scalarmult.Mult(privateKey, peersPublicKey)
  	salsa.HSalsa20(sharedKey, &zeros, sharedKey, &salsa.Sigma)
  	return sharedKey
  }
docs: // Precompute calculates the shared key between peersPublicKey and privateKey
  // and writes it to sharedKey. The shared key can be used with
  // OpenAfterPrecomputation and SealAfterPrecomputation to speed up processing
  // when using the same pair of keys repeatedly.