code: func parseInt64(d []byte) (n int64) {
  	if len(d) == 0 {
  		return -1
  	}
  	for _, dec := range d {
  		if dec < ascii_0 || dec > ascii_9 {
  			return -1
  		}
  		n = n*10 + (int64(dec) - ascii_0)
  	}
  	return n
  }
docs: // parseInt64 expects decimal positive numbers. We
  // return -1 to signal error