code: func (t *memTombstones) addInterval(ref uint64, itvs ...Interval) {
  	t.mtx.Lock()
  	defer t.mtx.Unlock()
  	for _, itv := range itvs {
  		t.intvlGroups[ref] = t.intvlGroups[ref].add(itv)
  	}
  }
docs: // addInterval to an existing memTombstones