code: public static <R, T> Function<T, R> spyRes(Function<T, R> function, Box<R> result) {
          return spy(function, result, Box.<T>empty());
      }
docs: Proxies a function spying for result.
  
  @param <R> the function result type
  @param <T> the function parameter type
  @param function the function that will be spied
  @param result a box that will be containing spied result
  @return the proxied function