docs: function(tree, initLevel, toLevel, action) {
      if (initLevel <= toLevel) {
        action(tree, initLevel);
        if(!tree.children) return;
        for ( var i = 0, ch = tree.children; i < ch.length; i++) {
          this.eachLevel(ch[i], initLevel + 1, toLevel, action);
        }
      }
    }