code: function (delta) {
              var choices = this.findHighlightableChoices(),
                  index = this.highlight();
  
              while (index > -1 && index < choices.length) {
                  index += delta;
                  var choice = $(choices[index]);
                  if (choice.hasClass("select2-result-selectable") && !choice.hasClass("select2-disabled") && !choice.hasClass("select2-selected")) {
                      this.highlight(index);
                      break;
                  }
              }
          }
docs: abstract