The way I implemented those "incompatibilities" currently is that merging a Fleeting and Persistent/Exile will cancel both effects on the resulting stone. But if you merge another arrow, then it adds Fleeting again. I think I prefer that it works the same for everything instead of having a different behavior for Exile compared to Persistent. But that means I need to nerf Eraser then. Maybe removing half of the opponent moves instead of all is a good compromise, it's still strong if played every turn but later opponents will also still give some challenge. Or I could instead give self damage to Eraser so it would be risky to spam it too often.