code: func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
  	return &encoder{
  		enc: enc,
  		w:   w,
  		buf: make([]byte, enc.base256BlockLen),
  		out: make([]byte, 128*enc.baseXBlockLen),
  	}
  }
docs: // NewEncoder returns a new baseX stream encoder.  Data written to
  // the returned writer will be encoded using enc and then written to w.
  // Encodings operate in enc.baseXBlockLen-byte blocks; when finished
  // writing, the caller must Close the returned encoder to flush any
  // partially written blocks.