Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  a : V.t  H0 : x2 = a
  ============================
   (x2 = a \/ VSet.In a _z_)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 6: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x2 = a \/ VSet.In a _z_
of type
constr of type tacvalue

Level 6: Backtrace:

Tactic failure.
Level 6: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.