code: function Edge(a, b) {
    var s = 0;
    function t() {}
    a._story(null, function() {
      if(s !== 0) {
        t.trigger();
      }
      s = 0;
    });
    b._story(null, function() {
      if(s !== 1) {
        t.trigger();
      }
      s = 1;
    });
    return Trigger(t);
  }
docs: check change between two states and triggers