@Override default CompletableFuture<Double> sumDouble(final ToDoubleFunction<? super T> fn){
  return CompletableFuture.supplyAsync(null,getExec());
}

@ Override default CompletableFuture < Double > sumDouble ( final ToDoubleFunction < ? super T > fn ) { return CompletableFuture . supplyAsync ( null , getExec ( ) ) ; }
perform an asynchronous sum operation
