code: function foldStyle(style) {
      for (var i = 0; i < style.length; i++) {
          for (var j = style.length - 1; j > i; j--) {
              if (style[j].filters.cloneWith(style[i].filters) === null) {
                  style.splice(j, 1);
              }
          }
      }
      return style;
  }
docs: Removes dead style definitions that can never be reached when filter-mode="first". The style is modified in-place and returned. The style must be sorted.