code: func (t *TombstoneGC) Hint(index uint64) {
  	expires := t.nextExpires()
  
  	t.Lock()
  	defer t.Unlock()
  	if !t.enabled {
  		return
  	}
  
  	// Check for an existing expiration timer and bump its index if we
  	// find one.
  	exp, ok := t.expires[expires]
  	if ok {
  		if index > exp.maxIndex {
  			exp.maxIndex = index
  		}
  		return
  	}
  
  	// Create a new expiration timer.
  	t.expires[expires] = &expireInterval{
  		maxIndex: index,
  		timer: time.AfterFunc(expires.Sub(time.Now()), func() {
  			t.expireTime(expires)
  		}),
  	}
  }
docs: // Hint is used to indicate that keys at the given index have been
  // deleted, and that their GC should be scheduled.