whenever we have multiple candidates we use fn try_merge_responses to try and merge them. This function is supposed to be complete.

We currently merge responses if they are all equal or alternatively if one of them has no external constraints.

This is quite fragile because it breaks the moment there are any region constraints as these sometimes contain '0: '0 constraints. We should remove those. We currently only opportunistically resolve region vars during canonicalization but the filtering should happen before then 🤔 It also does not work for any normalizes-to goal as their whole point is to constrain the term.

I feel like we should replace this with a proper subset check instead.