code: func (t *TombstoneGC) purgeBin(expires time.Time) uint64 {
  	t.Lock()
  	defer t.Unlock()
  
  	// Get the maximum index and clear the entry. It's possible that the GC
  	// has been shut down while this timer fired and got blocked on the lock,
  	// so if there's nothing in the map for us we just exit out since there
  	// is no work to do.
  	exp, ok := t.expires[expires]
  	if !ok {
  		return 0
  	}
  	delete(t.expires, expires)
  	return exp.maxIndex
  }
docs: // purgeBin gets the index for the given bin and then deletes the bin. If there
  // is no bin then this will return 0 for the index, which is ok.