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


DEBUG:Going to execute:
autorewrite with set_eq_simpl in *;
            VSetDecide.MSetDecideAuxiliary.inst_MSet_hypotheses; autorewrite
            with set_simpl set_eq_simpl in *; push not in * using
            MSet_decidability; VSetDecide.MSetDecideAuxiliary.substMSet;
            VSetDecide.MSetDecideAuxiliary.assert_decidability