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
           | H:context [ ?x = ?y :> ?T ]
             |- _ =>
                 tryif change T with V.t in H then 
                  fail else
                  tryif change T with VSet.t in H then fail else clear H  
           | H:?P
             |- _ =>
                 tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P)
                  holds by auto  100 with MSet_Prop then 
                  fail else clear H 
           end