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_empty> flags X1; elim H
Level 4: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := V.t
of type
constr of type tacvalue

Level 4: Backtrace:

Tactic failure.
Level 4: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.