code: func (s *MockStore) AtomicDelete(key string, previous *store.KVPair) (bool, error) {
  	mData := s.db[key]
  	if mData != nil && mData.Index != previous.LastIndex {
  		return false, types.BadRequestErrorf("atomic delete failed due to mismatched Index")
  	}
  	return true, s.Delete(key)
  }
docs: // AtomicDelete deletes a value at "key" if the key has not
  // been modified in the meantime, throws an error if this is the case