$assertAllow = new TestAssertion(true);
        $assertDeny = new TestAssertion(false);

        $acl = new Acl();
        $acl->addRole('staff');
        $acl->addResource('base');
        $acl->allow('staff', 'base', 'update', $assertAllow);

        $acl->addResource('user', 'base');
        $acl->allow('staff', 'user', 'update', $assertDeny);

        var_dump($acl->isAllowed('staff', 'user', 'update'));

class TestAssertion implements AssertionInterface
{
    protected $value;
    public function __construct($value)
    {
        $this->value = $value;
    }

    public function assert(Acl $acl, RoleInterface $role = null, ResourceInterface $resource = null, $privilege = null)
    {
        return $this->value;
    }
}