The other major computer symbolic algebra system (apart from Mathematica) which seems to have not quite as much traction because of… not having a messianic CEO? Having awful branding? It seems to be OK now that I look at it. In particular, it does what I expect regarding transforms of random variables.
Since everyone seems to know Mathematica, I guess I should describe it in terms of that? It is imperative-emphasis rather than functional emphasis; the upshot seems to be that if you want functional behaviour it has to be defined using the “inert” form?