the PR to remove TypeConstraint from the engine and the one to add UnionRule are both green! was going to merge the UnionRule one tomorrow morning in case anyone wants to review it, the TypeConstraint removal one still needs review. not blocking anything else though so no rush

I'd like to take another look at the UnionRule one if you don't mind :)

(should have just pinged you -- i will leave that for as long as it needs! i don't need it merged for anything else -- thanks!)