partial-map should never unify with fresh vars - this should be a hard error. This means partial-map functionality needs to be accessible by some other means - probably a constraint - `featurec` might be a good name for it. This way partial-maps never leak into an actual program, and users don't try to make the concrete data structure participate in unification. At the same time this does allow extracting features of interest via unification once the term (map) being checked actually becomes ground.