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

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