private Publisher<WriteResult> deleteByKey(StandardOperations.DeleteByKey op) {
      int deleted = 0;
      for(Object key: op.keys()) {
        if (store.remove(key) != null) {
          deleted++;
        }
      }

      return Flowable.just(WriteResult.empty().withDeletedCount(deleted));
    }
mapping between class and its store
criteria/inmemory/src/org/immutables/criteria/inmemory/InMemoryBackend.java