Goal:
    x : VSet.elt  p : VSet.t  H : ~ VSet.In x p
  ============================
   (VSet.Equal (VSet.remove x (VSet.add x p)) p)


DEBUG:Going to execute:
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)