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


DEBUG:Going to execute:
cbv beta zeta in *
Level 1: Backtrace:

Cannot find any non-recursive equality over x.
Level 1: Backtrace:

Cannot find any non-recursive equality over x.
Level 0: Backtrace:

Cannot find any non-recursive equality over x.