def maybe_face_bounding_box(detector, img):
    """
    Returns a bounding box if it finds
    ONE face. Any other case, it returns
    none

    Args:
        detector: dlib.get_front_face_detector()
        img: image
    """
    dets = detector(img, 1)
    if len(dets) >= 1:
        return dets
    return None