code: function(fn) {
          var elements = [];
          return this.each(function(el, i) {
              if (fn.call(el, i)) elements.push(el);
          }).set(elements);
      }
docs: filter
  ------
  
  Extend XUI with custom filters. This is an interal utility function, but is also useful to developers.
  
  ### syntax ###
  
  x$( window ).filter( fn );
  
  ### arguments ###
  
  - fn `Function` is called for each element in the XUI collection.
  
  `index` is the array index of the current element
  function( index ) {
  `this` is the element iterated on
  return true to add element to new XUI collection
  }
  
  ### example ###
  
  Filter all the `<input />` elements that are disabled:
  
  x$('input').filter(function(index) {
  return this.checked;
  });