+Note [The (~) TyCon)
+~~~~~~~~~~~~~~~~~~~~
+There is a perfectly ordinary type constructor (~) that represents the type
+of coercions (which, remember, are values). For example
+ Refl Int :: Int ~ Int
+
+Atcually it is not quite "perfectly ordinary" because it is kind-polymorphic:
+ Refl Maybe :: Maybe ~ Maybe
+
+So the true kind of (~) :: forall k. k -> k -> #. But we don't have
+polymorphic kinds (yet). However, (~) really only appears saturated in
+which case there is no problem in finding the kind of (ty1 ~ ty2). So
+we check that in CoreLint (and, in an assertion, in Kind.typeKind).
+
+Note [The State# TyCon]
+~~~~~~~~~~~~~~~~~~~~~~~