code: function buildPrevItem() {
          if ($body.classList.contains('overview')) {
              return false;
          }
          $curSlide = $slides[curIndex];
          var subBuilded = toArray($('.building'), $curSlide);
          var list;
          var buildingLen = subBuilded.length;
          var curList;
  
          if (buildingLen) {
              while (list = subBuilded.shift()) {
                  var clist = list.classList
                  clist.remove('building');
                  clist.add('tobuild');
                  curList = list;
                  if (clist.contains('subSlide')) {
                      var $item = toArray($('.subSlide.builded.subBuilded', $curSlide)).pop();
                      $item && $item.classList.remove('subBuilded');
                  }
              }
          }
          var builded = toArray($('.builded', $curSlide));
          if (!builded.length && !buildingLen) {
              return false;
          }
  
          var item = builded.pop();
          if (item) {
              if (!curList) {
                  curList = item;
              }
              list = item.classList;
              list.remove('builded');
              if (buildingLen === 0) {
                  list.add('tobuild');
                  item = builded.pop();
                  item.classList.remove('builded');
                  item.classList.add('building');
              } else {
                  list.add('building');
              }
  
          }
  
          $B.fire('slide do build', {
              slideID: curIndex,
              direction: 'prev',
              build: curList.dataset.index
          });
          $B.fire('slide.update', curIndex | 0, (curList.dataset.index | 0) + 1, 'prev');
          return true;
      }
docs: 单条往后走