Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt  _z_ : VSet.t  x0 : V.t  x1 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 _z_))
      (VSet.add x1 (VSet.add x0 _z_)))


DEBUG:Going to execute:
prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by auto 
100 with MSet_Prop