ViewStack: { 
 xml: "<div id='viewstack'/>",
 fun: function (sys, items, opts) {
  var args, children = this.children(),
   table = children.call("hide").hash(),
   ptr = table[opts.index] || children[0];
  if (ptr) ptr = ptr.trigger("show").show();
  this.on("switch", function ( e, to ) {
   table = this.children().hash();
   if ( !table[to] || table[to] == ptr ) return;
   e.stopPropagation();
   args = [].slice.call(arguments).slice(2);
   ptr.trigger("hide", [to+''].concat(args)).hide();
   ptr = table[to].trigger("show", [ptr+''].concat(args)).show();
  });
  return Object.defineProperty({}, "selected", { get: function() {return ptr;}});
 }
}

Example1: {
 xml: `<ViewStack index='bar'>
    <button id='foo'>foo</button>
    <button id='bar'>bar</button>
   </ViewStack>`
}

Example2: {
 xml: "<ViewStack id='viewstack'>    <button id='foo'>foo</button>    <button id='bar'>bar</button>   </ViewStack>"
 fun: function (sys, items, opts) {
  sys.viewstack.on("click", "*", function(e) {
   var to = this + '' == "foo" "bar" : "foo",
    data = "hello, world";
   this.trigger("switch", [to, data]);
  });
  sys.foo.on("show", function (e, prev, data) {
   console.log("previous page is " + prev, "from data is " + data);
  });
  sys.bar.on("hide", function (e, prev, data) {
   console.log("previous page is " + prev, "from data is " + data);
  });
 }
}

Example3: {
 xml: "<ViewStack id='viewstack'>    <button id='foo'>foo</button>   </ViewStack>"
 fun: function (sys, items, opts) {
  var xml = "<button id='bar'>bar</button>";
  sys.viewstack.append(xml).trigger("switch", "bar");
 }
}

Example4: {
 xml: `<ViewStack>
    <button id='foo'>foo</button>
    <button id='bar'>bar</button>
    <button id='alice'>alice</button>
   </ViewStack>`
 map: { defer: "bar alice" }
}

Example5: {
 xml: `<ViewStack id='example'>
    <button id='foo'>foo</button>
    <button id='bar'>bar</button>
    <button id='alice'>alice</button>
   </ViewStack>`,
 fun: function (sys, items, opts) {
  sys.example.on("show", "button", function (e, prev) {
   window.history.pushState({name: this + ""}, null, "/" + this);
  });
  window.addEventListener("popstate", function(e) {
   sys.example.trigger("switch", e.state.name);
  });
 }
}