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:
idtac
Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.