Goal:
    x : VSet.elt  y : V.t  p : VSet.t  H : x <> y  a : V.t
  ============================
   (VSet.In a (VSet.add x (VSet.remove y p)) <->
    VSet.In a (VSet.remove y (VSet.add x p)))


DEBUG:Going to execute:
progress change T with V.t in J
DEBUG LTAC Evaluated term: VSet.elt
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.