code: public static <T1, T2, R> BiFunction<T1, T2, R> spyRes(BiFunction<T1, T2, R> function, Box<R> result) {
          return spy(function, result, Box.<T1>empty(), Box.<T2>empty());
      }
docs: Proxies a binary function spying for result.
  
  @param <T1> the function first parameter type
  @param <T2> the function second parameter type
  @param <R> the function result type
  @param function the function that will be spied
  @param result a box that will be containing spied result
  @return the proxied function