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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def keep_only_positive_boxes(boxes):
 """ Given a set of BoxList containing the `labels` field, return a set of BoxList for which `labels > 0`. Arguments: boxes (list of BoxList) """

    assert isinstance(boxes, (list, tuple))
    assert isinstance(boxes[0], BoxList)
    assert boxes[0].has_field("labels")
    positive_boxes = []
    positive_inds = []
    num_boxes = 0
    for boxes_per_image in boxes:
        labels = boxes_per_image.get_field("labels")
        inds_mask = labels > 0
        inds = inds_mask.nonzero().squeeze(1)
        positive_boxes.append(boxes_per_image[inds])
        positive_inds.append(inds_mask)
    return positive_boxes, positive_inds