Description: The result of a function value is always a subset of the union of the range, even if it is invalid and thus empty. (Contributed by Stefan O'Rear, 2-Nov-2014) (Revised by Mario Carneiro, 31-Aug-2015) (Proof shortened by SN, 13-Jan-2025)
|- ( F ` X ) C_ U. ran F
|- ( x e. ( F ` X ) -> x e. U. ran F )