Goal:
    x : V.t  s : VSet.t  y : VSet.elt
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x 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