code: @Override
    public List<CompletableFuture<Void>> mutate(List<? extends Mutation> mutations) {
      return helper
          .mutate(mutations)
          .stream()
          .map(listenableFuture ->
              toCompletableFuture(listenableFuture)
                  .thenApply(r-> (Void) null))
          .collect(Collectors.toList());
    }
docs: {@inheritDoc}