If I remember correctly, I changed those fields to be sets as order of the symbols was not specified or required in some cases.

Can you share an example where this would help?

It may be the case that order is not required but sets are particularly annoying to work with from a user perspective. For example, If I have a system and I want to access a particular constant, I can't simply index into the set or I can't pop it (cause I get an arbitrary symbol), etc. I find that the only time I ever use sets is for a quick conversion to one to use unions/intersections/etc or to make a list unique. Other than that they are not friendly for manipulation like lists and tuples are. My pydy related code these days is full of list(sys.constants_symbols)[2] and list(sys.specifieds_symbols)[2], for example.

I don't feel too strongly about keeping sets and would be happy to review a PR.

Ok, thanks. I'll probably be posting a number of PRs over winter break. I am just finishing teaching with sympy/pydy in grad level multibody dynamics and a number of things came up that I want to fix. The most important is getting your pythreejs pr finished up so that we can use Jupyter >= 5.0.