code: function filter(selector) {
    var arr = [], $ = this.air;
    this.each(function(el) {
      var selections;
      if(typeof selector === 'function') {
        if(selector.call(el)) {
          arr.push(el);
        }
      }else{
        selections = $(selector);
        if(~selections.dom.indexOf(el)) {
          arr.push(el);
        }
      }
    });
    return $(arr);
  }
docs: Reduce the set of matched elements to those that match the selector
  or pass the function's test.