+ -- type signature \ x::a -> e
+
+ | ExistTv -- An existential type variable bound by a pattern for
+ -- a data constructor with an existential type. E.g.
+ -- data T = forall a. Eq a => MkT a
+ -- f (MkT x) = ...
+ -- The pattern MkT x will allocate an existential type
+ -- variable for 'a'. We distinguish these from all others
+ -- on one place, namely InstEnv.lookupInstEnv.