@Override public <U>CompletableFuture<U> foldLeft(final U seed,final BiFunction<U,? super T,U> function){
  return CompletableFuture.supplyAsync(null,exec);
}

@ Override public < U > CompletableFuture < U > foldLeft ( final U seed , final BiFunction < U , ? super T , U > function ) { return CompletableFuture . supplyAsync ( null , exec ) ; }
perform an asynchronous foldleft operation
