| (C.Abst _, _) | (_, C.Abst _) | (C.Cast _, _) | (_, C.Cast _)
| (C.Implicit, _) | (_, C.Implicit) ->
raise (Impossible 3) (* we don't trust our whd ;-) *)
| (C.Abst _, _) | (_, C.Abst _) | (C.Cast _, _) | (_, C.Cast _)
| (C.Implicit, _) | (_, C.Implicit) ->
raise (Impossible 3) (* we don't trust our whd ;-) *)