This turned out to be a weird "match" the lines game. My initial idea was to have some initial expression change into something else, but it ended up almost going off topic/theme. Furthermore, it turns out that there were not that many interesting expressions off the top of my head that didn't include sin/cos, which made it more difficult to come up with puzzles beyond the "tutorial" phase. You could start piling and nesting functions on top of each other but then it would look quite arbitrary.

The game itself is straighforward. You select level 1 and it'll give you some directions over the first few levels. The main idea is that you edit the expression of the blue line, that must match with the expression of the red line. Expressions are compared by sampling and checking the distance, so they don't need to match exactly, and don't even need to be the same as long as they evaluate to the same thing (+/- a small margin).