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


DEBUG:Going to execute:
<Coq.Init.Tauto.find_cut>
            ltac:(fun id0 id1 X2 =>
                    assert X2; [ exact (id0 id1) | clear id0 ])
Level 5: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.In a _z_
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.
Level 4: Backtrace:

Tactic failure.
Level 4: Backtrace:

Tactic failure.