code: func New(alphabet []byte) *Encoding {
  	enc := &Encoding{}
  	copy(enc.alphabet[:], alphabet[:])
  	for i := range enc.decodeMap {
  		enc.decodeMap[i] = -1
  	}
  	for i, b := range enc.alphabet {
  		enc.decodeMap[b] = int64(i)
  	}
  	return enc
  }
docs: // New creates a new base58 encoding.