Goal:
  
  x : V.t
  s : VSet.t
  s' : VSet.t
  H : VSetProp.Add x s s'
  z : VSet.elt
  Hz : VSet.In z s
  ============================
   (x = z)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 4: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := V.t
of type
constr of type tacvalue

Level 4: Backtrace:

Tactic failure.
Level 4: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.