Goal:
    x : V.t  p : VSet.t  H : VSet.In x p -> False  a : V.t  H0 : x = a
  ============================
   (((x = a \/ VSet.In a p) /\ x <> a -> VSet.In a p) /\
    (VSet.In a p -> (x = a \/ VSet.In a p) /\ x <> a))


DEBUG:Going to execute:
<Coq.Init.Tauto.find_cut>
            ltac:(fun id0 id1 X2 =>
                    assert X2; [ exact (id0 id1) | clear id0 ])
Level 3: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = a
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.