Goal:
    x : V.t  s : VSet.t  y : VSet.elt
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x s)))


DEBUG:Going to execute:
fail "no need to abstract a variable"
Level 2: evaluation returns
<coq-core.plugins.ltac::is_hyp@0>
where
$1 := x
of type
constr of type tacvalue