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:
fail
DEBUG LTAC Evaluated term: VSet.t
DEBUG LTAC Evaluated term: Type
Level 3: type of P
 has value 
Type