Description:
------------
Please allow for subtype polymorphism/downcasting in interfaces. 

Test script:
---------------
interface Deliverable {}

interface DeliverableWithTruck extends Deliverable {}

interface DeliveryService {
    function deliver( Deliverable $deliverable, $destination );
}


class CookieJar implements DeliverableWithTruck {}

class ExpressDeliveryService implements DeliveryService {

    function deliver( DeliverableWithTruck $deliverable, $destination ) {
        echo "Delivery underway";
    }

}


$fedEx = new ExpressDeliveryService;
$chocolateChipCookies = new CookieJar;

$fedEx->deliver( $chocolateChipCookies, "Sesame Street" );


Expected result:
----------------
Delivery underway

Actual result:
--------------
Fatal error: Declaration of ExpressDeliveryService::deliver() must be compatible 
with that of DeliveryService::deliver()