Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x = y -> False
  a : V.t
  H0 : y = a
  H1 : VSet.In a p
  H2 : False
  ============================
   (x = a \/ VSet.In a p)


DEBUG:Going to execute:
<Coq.Init.Tauto.flatten_contravariant_disj> flags X1 X2 id
Level 3: evaluation returns
<coq-core.plugins.tauto::flatten_contravariant_conj@0>
where
tauto_flags := 
of type
tauto_flags
id := H
of type
constr
X2 := False
of type
constr
X1 := x = y
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.