def white_strip(q, f, x, add_v):
    """
    Subroutine of ``all_intervals_covered()``.

    Check if column and diagonal strip of x are all white.
    If so, x cannot be covered. Vertex function is not extreme.
    """
    for xx in [x, (f - x - 1) % q]:
        for y in range(1, q):
            for u in [xx, (xx - y) % q]:
                if add_v[u, y] and add_v[u + 1, y]:
                    return False
        for y in (list(range(xx)) + list(range(xx + 1, q))):
            if add_v[xx, y + 1] and add_v[xx + 1, y] and (xx + y + 1) % q != f:
                return False
    return True
