Hi,
I have no strong opinion and I don't think it matters much either way.
If we come up with an ingenious choice later (before the first release,
obviously), renaming is always a possiblity.
I agree ct is too short. Imho it shouldn't be too long either (because
then I have difficulty thinking the module name in my head -- not sure
if this explanation makes sense or is even relevant).
My personal favorite would be categories, because it is in keeping with
other sympy module names. This would suggest the unconventional
"manifolds" for the differential geometry module.
In any case, my suggestion (to both of you) is to not waste much thought
on the module name. It is used very seldomly in code anyway (because in
sympy we tend to "import the leaves").
Best,
Tom