Goal:
    x : V.t  s : VSet.t
  ============================
   (~ VSet.In x (VSet.remove 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