code: func (r *Reader) ReadUint64() uint64 {
  	if r.err != nil {
  		return 0
  	}
  	var n int
  	n, r.err = io.ReadFull(r.rd, r.b[:8])
  	r.cnt += n
  	if r.err != nil {
  		return 0
  	}
  	return binary.LittleEndian.Uint64(r.b[:8])
  }
docs: // ReadUint64 reads and returns an uint64.