Goal:
  
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  x3 : V.t
  a : V.t
  H : VSet.In a _z_
  ============================
   (x1 = a \/ x2 = a \/ x3 = a \/ VSet.In a _z_)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 5: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.t
of type
constr of type tacvalue

Level 5: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.
Level 4: Backtrace:

Tactic failure.
Level 4: Backtrace:

Tactic failure.