Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : V.t
  _b_ : Z
  xt : V.t
  _z_ : VSet.t
  ============================
   (VSet.Equal _z_ _z_)


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