code: func NewShake256() ShakeHash {
  	if h := newShake256Asm(); h != nil {
  		return h
  	}
  	return &state{rate: 136, dsbyte: 0x1f}
  }
docs: // NewShake256 creates a new SHAKE256 variable-output-length ShakeHash.
  // Its generic security strength is 256 bits against all attacks if
  // at least 64 bytes of its output are used.