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:   };
  
    //-----------------------------------------
    // refresh [<] avatar [>]
    //-----------------------------------------
  
    const refreshAvatarControls = () => {
  
negative.code: 
    const prevEventIndex = () => {
      const n = param.nowIndex;
      const firstIndex = 1;
      if (n === null || n === firstIndex) {
        return null;
      } else {
        return n - 1;
  