Does an in-place addition of the four points that can be composed by
    combining coordinates from the two lists to the given list of anchors

def pairwise_point_combinations(xs, ys, anchors):
    """
    Does an in-place addition of the four points that can be composed by
    combining coordinates from the two lists to the given list of anchors
    """
    for i in xs:
        anchors.append((i, max(ys)))
        anchors.append((i, min(ys)))
    for i in ys:
        anchors.append((max(xs), i))
        anchors.append((min(xs), i))