Could anyone explain the last four lines (lt, rb, inter, and the return expression) of the following code ? I do not quite understand how : works inside [] for pytorch
def box_iou(boxes1, boxes2):
    # https://github.com/pytorch/vision/blob/master/torchvision/ops/boxes.py
    """
    Return intersection-over-union (Jaccard index) of boxes.
    Both sets of boxes are expected to be in (x1, y1, x2, y2) format.
    Arguments:
        boxes1 (Tensor[N, 4])
        boxes2 (Tensor[M, 4])
    Returns:
        iou (Tensor[N, M]): the NxM matrix containing the pairwise
            IoU values for every element in boxes1 and boxes2
    """

    def box_area(box):
        # box = 4xn
        return (box[2] - box[0]) * (box[3] - box[1])

    area1 = box_area(boxes1.t())
    area2 = box_area(boxes2.t())

    lt = torch.max(boxes1[:, None, :2], boxes2[:, :2])  # [N,M,2]
    rb = torch.min(boxes1[:, None, 2:], boxes2[:, 2:])  # [N,M,2]

    inter = (rb - lt).clamp(min=0).prod(2)  # [N,M]
    return inter / (area1[:, None] + area2 - inter)  # iou = inter / (area1 + area2 - inter)