Julia's Symbolics.jl allows users to easily add custom rewrite rules. However, it's not a state-of-the-art CAS. I don't have that much experience with others. Mathematica doesn't allow it, I think.
Are fundamental constants other-valued in any Many Worlds interpretations, or are e, i, and Pi always e, i, and pi with the same relations?
Countability and continuua (in a Hilbert space of degree n, where n is or is not inconstant like the many forms of [quantum discord] entropy and the energy that represents them)
TIL the separable states problem is considered NP-hard, and many models specify independence of observation as necessary.
> In this game, you get own version of the natural numbers, called `mynat`, in an interactive theorem prover called Lean. Your version of the natural numbers satisfies something called the principle of mathematical induction, and a couple of other things too (Peano's axioms).
Such axioms are hard-coded in SymPy, SageMath, and Mathematica but not in Lean Mathlib (which is not optimized for performance)
If there are an infinite number of unitary transformations (as rotations on a Bloch sphere for example), I find it unlikely that we've yet discovered all of the requisite operators and axioms and coded them into any human CAS that exists in present day.