code: func (r *PrevRevisions) Del(key string) (found bool, prev datasync.KeyVal) {
  	found, prev = r.Get(key)
  
  	if found {
  		r.mu.Lock()
  		delete(r.revisions, key)
  		r.mu.Unlock()
  	}
  
  	return found, prev
  }
docs: // Del deletes the entry from revisions and returns previous value.