Tip: How did you solve level 9?
On L9 I could change what WALL IS, but I can't do that on L12. They're up against the side, so there's no way to change WALL IS STOP, FLAG IS WIN, or BABA IS YOU. All I have to work with are ICE, SLIP, and EMPTY -- and all I can make them IS is WALL, STOP, BABA, YOU, or each other. Another tip? Sorry that I'm dense.