Goal:
  
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  i : V.t
  H1 : VSet.In i s
  H2 : VSet.In i s''
  H3 : VSet.In i s''
  H0 : VSet.In i s'
  H4 : False
  ============================
   (VSet.In i s'')


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.