The type parameter is a phantom type that represents the sort.
val equal : 'sort1t->'sort2t->(('sort1, 'sort2)eq, ('sort1, 'sort2)eq->'any) Stdlib.Either.t
Decides the equality of two sorts. Iff the sorts are equal, returns a proof that their types are equal. Iff the sorts are unequal, it returns a proof that their types are not equal.