code: func (r *bufferingReaderAt) grabBuf() []byte {
  	if len(r.pool) != 0 {
  		b := r.pool[len(r.pool)-1]
  		r.pool = r.pool[:len(r.pool)-1]
  		return b
  	}
  	return make([]byte, r.blockSize)
  }
docs: // grabBuf returns a byte slice of blockSize size.