public function newCart(PhabricatorUser $actor, PhortuneCartImplementation $implementation, PhortuneMerchant $merchant)
{
$cart = PhortuneCart::initializeNewCart($actor, $this, $merchant);
$cart->setCartClass(get_class($implementation));
$cart->attachImplementation($implementation);
$implementation->willCreateCart($actor, $cart);
return $cart->save();
}