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


DEBUG:Going to execute:
repeat
            match goal with
            | H:context [ ?x <> ?y ]
              |- _ => assert new (x = y \/ x <> y) by
              apply VSetDecide.MSetDecideAuxiliary.dec_eq
            | H:context [ ~ VSet.In ?x ?s ]
              |- _ => assert new (VSet.In x s \/ ~ VSet.In x s) by
              apply VSetDecide.MSetDecideAuxiliary.dec_In
            | |- context [ ?x <> ?y ] => assert new 
              (x = y \/ x <> y) by
              apply VSetDecide.MSetDecideAuxiliary.dec_eq
            | |- context [ ~ VSet.In ?x ?s ] => assert new
              (VSet.In x s \/ ~ VSet.In x s) by
              apply VSetDecide.MSetDecideAuxiliary.dec_In
            end;
            repeat match goal with
                   | _:~ ?P, H:?P \/ ~ ?P |- _ => clear H
                   end