code: func (tf tFiles) lessByKey(icmp *iComparer, i, j int) bool {
  	a, b := tf[i], tf[j]
  	n := icmp.Compare(a.imin, b.imin)
  	if n == 0 {
  		return a.fd.Num < b.fd.Num
  	}
  	return n < 0
  }
docs: // Returns true if i smallest key is less than j.
  // This used for sort by key in ascending order.