Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t  H0 : x = a
  ============================
   (x = a)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 6: 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 6: Backtrace:

Tactic failure.
Level 6: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.
Level 5: Backtrace:

Tactic failure.