anchor.diff:      $.each($('.diff-traffic-light', trafficLights), (_, light) => {
         const $light = $(light);
         const nowIndex = $light.data('index');
  -      // review() function is also used by < event-index > navigator
  +      // review() is also used by [<] event-index [>] navigator
         param.events[nowIndex].review = () => {
           param.wasIndex = inDiffMode() ? wasIndex : nowIndex;
           param.nowIndex = nowIndex;
  
positive.code: 
    const prevEventIndex = () => {
      const n = param.nowIndex;
      if (n === null || n === firstIndex()) {
        return null;
      } else {
        return n - 1;
  
negative.code:   };
  
    //-----------------------------------------
    // refresh [<] event-index [>]
    //-----------------------------------------
    // [<] is enabled if there is an event before param.wasIndex
    // [>] is enabled if there is an event after  param.nowIndex
    //-----------------------------------------
  
    const refreshEventControls = () => {
  