Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  x3 : V.t  a : V.t
  ============================
   (VSet.In a (VSet.add x0 (VSet.add x1 (VSet.add x2 (VSet.add x3 _z_)))) <->
    VSet.In a (VSet.add x2 (VSet.add x3 (VSet.add x0 (VSet.add x1 _z_)))))


DEBUG:Going to execute:
subst x
Level 1: Backtrace:

Cannot find any non-recursive equality over x2.
Level 1: Backtrace:

Cannot find any non-recursive equality over x2.