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


DEBUG:Going to execute:
fun t =>
             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)