Goal:
  
  x : V.t
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  da : VSet.Subset s s'
  i : VSet.elt
  disj : ~ VSet.In i (VSet.inter s' s'')
  ininter : VSet.In i (VSet.inter s s'')
  ============================
   (VSet.In i (VSet.inter s' s''))


DEBUG:Going to execute:
repeat
            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