Needs just one more thing

My favorite game from "back in the day," when I used to buy puzzle books for vacations. It would be nice, however, if there was a way, on more challenging puzzles, to indicate that I am just trying out my possibilities - maybe turning the numbers red, or just circling the possibility I am running. That way, when I determine that it wont work, I know which numbers can be erased, without losing the numbers I spent a lot of time confirming are correct. Otherwise, a fun challenge to kill time.

Geutz about Real Kakuro™