Goal:
    x : V.t  s : VSet.t  y : V.t  H0 : x = y -> False  H1 : VSet.In y s
  ============================
   (False -> x = y \/ VSet.In y s /\ x <> y)


DEBUG:Going to execute:
intro; clear id
Level 3: evaluation returns
<coq-core.plugins.ltac::cut@0>
where
$1 := False
of type
constr of type tacvalue