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

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