code: func newEncoding(encoder string) *encoding {
  	e := new(encoding)
  	e.encode = encoder
  	for i := 0; i < len(e.decode); i++ {
  		e.decode[i] = 0xff
  	}
  	for i := 0; i < len(encoder); i++ {
  		e.decode[encoder[i]] = byte(i)
  	}
  	return e
  }
docs: // newEncoding constructs a new encoding defined by the given alphabet,
  // which must be a 32-byte string.