Goal:
  
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  i : V.t
  disj : VSet.In i s' -> ~ VSet.In i s''
  H : VSet.In i s -> VSet.In i s'
  H1 : VSet.In i s
  H2 : VSet.In i s''
  H3 : ~ VSet.In i s''
  ============================
   (VSet.In i s' /\ VSet.In i s'')


DEBUG:Going to execute:
PTRDEBUGTAC<Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro; clear id
Level 3: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := VSet.In i s''
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.