code: function(number) {
      var layers = {}
      var c = 0;
      for(var i = 0; i < this.layers.length; ++i) {
        var layer = this.layers[i];
        layers[i] = c;
        if(layer.options && !layer.options.hidden) {
          ++c;
        }
      }
      return layers[number];
    }
docs: given number inside layergroup returns the real index in tiler layergroup`