Goal:
    x : VSet.elt  y : VSet.elt  p : VSet.t  H : x <> y
  ============================
   (VSet.Equal (VSet.add x (VSet.remove y p)) (VSet.remove y (VSet.add x p)))


DEBUG:Going to execute:
Ltac.is_var t
Level 2: evaluation returns
tryif Ltac.is_var t then fail "no need to abstract a variable" else
 (let x := fresh "x" in
  set (x := t) in *; try clearbody x) 
where
t := y
of type
constr of type tacvalue