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


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; Ltac.split
Level 3: evaluation returns
<coq-core.plugins.tauto::flatten_contravariant_disj@0>
where
tauto_flags := 
of type
tauto_flags
id := H2
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.