Goal:
    x : V.t  s : VSet.t  y : V.t  H1 : x = y  H : False
  ============================
   (x = y \/ VSet.In y s)


DEBUG:Going to execute:
elim H
Level 4: evaluation returns
<coq-core.plugins.tauto::is_empty@0>
where
tauto_flags := 
of type
tauto_flags
X1 := False
of type
constr of type tacvalue