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


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear id
Level 3: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.t
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.