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


DEBUG:Going to execute:
match goal with
           | H:VSet.In ?x ?r -> False
             |- VSet.In ?x ?s -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:VSet.In ?x ?r -> False
             |- ?x = ?y -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:VSet.In ?x ?r -> False
             |- ?y = ?x -> False =>
                 contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
           | H:?P -> False
             |- ?Q -> False =>
                 tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_elt_Prop P)
                  holds by auto  100 with MSet_Prop then
                  (contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body)
                   else VSetDecide.MSetDecideAuxiliary.fsetdec_body 
           | |- _ => VSetDecide.MSetDecideAuxiliary.fsetdec_body
           end