code: func NewUnionStore(snapshot Snapshot) UnionStore {
  	return &unionStore{
  		BufferStore:        NewBufferStore(snapshot, DefaultTxnMembufCap),
  		snapshot:           snapshot,
  		lazyConditionPairs: make(map[string]*conditionPair),
  		opts:               make(map[Option]interface{}),
  	}
  }
docs: // NewUnionStore builds a new UnionStore.