function format(str, attrs) {
    for(var i = 1; i < arguments.length; ++i) {
      var attrs = arguments[i];
      for(var attr in attrs) {
        str = str.replace(RegExp('\\{' + attr + '\\}', 'g'), attrs[attr]);
      }
    }
    return str;
  }
`coord` object like {x : tilex, y: tiley }
`zoom` quadtree zoom level