Goal:
    x : V.t  s : VSet.t  y : V.t  H0 : x = y
  ============================
   ((x = y \/ VSet.In y s -> x = y \/ VSet.In y s /\ x <> y) /\
    (x = y \/ VSet.In y s /\ x <> y -> x = y \/ VSet.In y s))


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; Ltac.split
Level 3: 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 3: Backtrace:

Tactic failure: No matching clause.
Level 3: Backtrace:

Tactic failure: No matching clause.
Level 3: Backtrace:

Tactic failure: No matching clause.
Level 2: Backtrace:

Tactic failure: No matching clause.