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:
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