code: func (m *Multisig) Key(seed Trytes, index uint64, security SecurityLevel, spongeFunc ...SpongeFunction) (Trytes, error) {
  	h := GetSpongeFunc(spongeFunc, kerl.NewKerl)
  
  	subseed, err := signing.Subseed(seed, index, h)
  	if err != nil {
  		return "", err
  	}
  
  	keyTrits, err := signing.Key(subseed, security, h)
  	if err != nil {
  		return "", err
  	}
  
  	return MustTritsToTrytes(keyTrits), nil
  }
docs: // Key gets the key value of a seed.