docs: function(ev, attr, how, newVal, oldVal) {
                      // when a change happens, create the named event.
                      can.batch.trigger(this, {
                              type: attr,
                              batchNum: ev.batchNum
                          }, [newVal, oldVal]);
  
                      if (how === "remove" || how === "add") {
                          can.batch.trigger(this, {
                                  type: "__keys",
                                  batchNum: ev.batchNum
                              });
                      }
                  }