Goal:
    s' : VSet.t  t0 : VSet.t  a : V.t  H : VSet.In a s'
  ============================
   (VSet.In a (VSet.union t0 s'))


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