@Override public CompletableFuture<Optional<T>> reduce(final BinaryOperator<T> accumulator){
  return CompletableFuture.supplyAsync(null);
}

@ Override public CompletableFuture < Optional < T > > reduce ( final BinaryOperator < T > accumulator ) { return CompletableFuture . supplyAsync ( null ) ; }
asynchronously perform a stream reduction
