Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  x3 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 (VSet.add x2 (VSet.add x3 _z_))))
      (VSet.add x2 (VSet.add x3 (VSet.add x0 (VSet.add x1 _z_)))))


DEBUG:Going to execute:
unfold VSet.Empty, VSet.Subset, VSet.Equal in *
Level 1: Backtrace:

_z_ is used in conclusion.
Level 1: Backtrace:

_z_ is used in conclusion.
Level 1: Backtrace:

_z_ is used in conclusion.
Level 0: Backtrace:

_z_ is used in conclusion.