code: func (us *unionStore) markLazyConditionPair(k Key, v []byte, e error) {
  	us.lazyConditionPairs[string(k)] = &conditionPair{
  		key:   k.Clone(),
  		value: v,
  		err:   e,
  	}
  }
docs: // markLazyConditionPair marks a kv pair for later check.
  // If condition not match, should return e as error.