function __construct($update_url = null)
{
/**
* requested_attributes: The attributes that have been
* requested thus far, indexed by the type URI.
*/
$this->requested_attributes = array();
/**
* update_url: A URL that will accept responses for this
* attribute exchange request, even in the absence of the user
* who made this request.
*/
$this->update_url = $update_url;
}