Goal:
    x : VSet.elt  y : VSet.elt  p : VSet.t
  ============================
   (VSet.Equal (VSet.add x (VSet.add y p)) (VSet.add y (VSet.add x p)))


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