code: public static By callback(final Function<SearchContext, List<WebElement>> callback) {
  		if (callback == null)
  			throw new IllegalArgumentException("Cannot find elements when the callback is null");
  
  		return new ByCallback(callback);
  	}
docs: Finds elements via custom callback which accepts {@link SearchContext}
  and returns {@link List} of {@link WebElement}s.
  
  @param callback callback
  @return a By which locates elements by callback.