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 \/ VSet.In a _z_)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; Ltac.split
Level 6: evaluation returns
<coq-core.plugins.tauto::find_cut@0>
where
k := fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]
     where
     flags := 
     of type
     tauto_flags
of type
tacvalue of type tacvalue

Level 6: Backtrace:

Tactic failure: No matching clause.
Level 6: Backtrace:

Tactic failure: No matching clause.
Level 6: Backtrace:

Tactic failure: No matching clause.
Level 5: Backtrace:

Tactic failure: No matching clause.