code: @Nullable
      public <R> R custom(@NotNull Function<Exceptional<T>, R> function) {
          Objects.requireNonNull(function);
          return function.apply(this);
      }
docs: Applies custom operator on {@code Exceptional}.
  
  @param <R> the type of the result
  @param function  a transforming function
  @return a result of the transforming function
  @throws NullPointerException if {@code function} is null
  @since 1.1.9