code: public static NumberBinding sum(final ObservableList<? extends Number> numbers) {
          return Bindings.createDoubleBinding(() -> numbers.stream().mapToDouble(Number::doubleValue).sum(), numbers);
      }
docs: Creates a number binding that contains the sum of the numbers of the given observable list of numbers.
  
  @param numbers the observable list of numbers.
  
  @return a number binding.