whenEmpty

{
    static auto arrA = [1, 2, 3];
    auto r = arrA.whenEmpty!((){ arrA = null; });
    assert(equal(r, arrA));
    assert(arrA is null);
}
{
    static auto arrB = [1, 2, 3];
    auto r = arrB.whenEmpty!((){ arrB = null; });
    //assert(equal(r, arr));
    r.popFront(); r.popFront();
    assert(arrB !is null);
    r.popFront();
    assert(arrB is null);
}