Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x = y -> False
  a : V.t
  H0 : y = a
  H2 : x = a
  ============================
   (False -> x = a \/ VSet.In a p)


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