In my downtime, I'm writing a CAS in Python. It has to be from scratch, because I want it to fit on my calculator, and all of the solutions out there import a lot of data. Currently, there are two separate programs, one which can do full polynomial arithmetic, but you have to type in the polynomial in program form, as a list of coefficients starting from x^0. I'm working on merging it with the CAS interface, which will evaluate expressions recursively. For example: (sin x)^2 is sin inputted into the function f(y) = y^2. Though I don't have time to make this logic perfect, it will be interesting to learn how to program a u-substitution for arbitrary input.