+-- | Injective 'TyCon's can be decomposed, so that
+-- T ty1 ~ T ty2 => ty1 ~ ty2
+isInjectiveTyCon :: TyCon -> Bool
+isInjectiveTyCon tc = not (isSynTyCon tc)
+ -- Ultimately we may have injective associated types
+ -- in which case this test will become more interesting
+ --
+ -- It'd be unusual to call isInjectiveTyCon on a regular H98
+ -- type synonym, because you should probably have expanded it first
+ -- But regardless, it's not injective!
+