Goal:
  
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  x : V.t
  hadd : VSetProp.Add x s s'
  hin : ~ VSet.In x s
  H : ~ VSet.In x s''
  ============================
   (~ VSet.In x (VSet.union s s''))


DEBUG:Going to execute:
repeat
            match goal with
            | |- context [ VSet.singleton ?t ] =>
                  VSetDecide.MSetDecideAuxiliary.abstract_term t
            | _:context [ VSet.singleton ?t ]
              |- _ => VSetDecide.MSetDecideAuxiliary.abstract_term t
            | |- context [ VSet.add ?t _ ] =>
                  VSetDecide.MSetDecideAuxiliary.abstract_term t
            | _:context [ VSet.add ?t _ ]
              |- _ => VSetDecide.MSetDecideAuxiliary.abstract_term t
            | |- context [ VSet.remove ?t _ ] =>
                  VSetDecide.MSetDecideAuxiliary.abstract_term t
            | _:context [ VSet.remove ?t _ ]
              |- _ => VSetDecide.MSetDecideAuxiliary.abstract_term t
            | |- context [ VSet.In ?t _ ] =>
                  VSetDecide.MSetDecideAuxiliary.abstract_term t
            | _:context [ VSet.In ?t _ ]
              |- _ => VSetDecide.MSetDecideAuxiliary.abstract_term t
            end