-isCoercionKind :: Kind -> Bool
--- All coercions are of form (ty1 :=: ty2)
--- This function is here rather than in Coercion,
--- because it's used by substTy
-isCoercionKind k | Just k' <- kindView k = isCoercionKind k'
-isCoercionKind (PredTy (EqPred {})) = True
-isCoercionKind other = False
-