code: func (r *buffer) readLimited(n int, src *countReader) error {
  	if r.hasData() {
  		panic("bgzf: read into non-empty buffer")
  	}
  	r.off = 0
  	var err error
  	r.size, err = io.ReadFull(src, r.data[:n])
  	return err
  }
docs: // readLimited reads n bytes into the buffer from the given source.