code: func (s *Stats) Sub(other *Stats) Stats {
  	if other == nil {
  		return *s
  	}
  	var diff Stats
  	diff.FreePageN = s.FreePageN
  	diff.PendingPageN = s.PendingPageN
  	diff.FreeAlloc = s.FreeAlloc
  	diff.FreelistInuse = s.FreelistInuse
  	diff.TxN = s.TxN - other.TxN
  	diff.TxStats = s.TxStats.Sub(&other.TxStats)
  	return diff
  }
docs: // Sub calculates and returns the difference between two sets of database stats.
  // This is useful when obtaining stats at two different points and time and
  // you need the performance counters that occurred within that time span.