Goal:
  
  x : V.t
  s : VSet.t
  s' : VSet.t
  z : VSet.elt
  H : VSetProp.Add x s (VSet.remove z s')
  H0 : VSet.In x s -> False
  i : VSet.elt
  H2 : VSet.In i s
  H3 : VSet.E.eq i z -> False
  ============================
   (VSet.In i s)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@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.