def _compute_positive_examples(
        label,
        label_to_items,
        label_graph
    ):
    """
    Compute the positive items for a given label.
    This set consists of all items labelled with a 
    descendent of the label
    """
    positive_items = label_to_items[label].copy()
    desc_labels = label_graph.descendent_nodes(label)
    for desc_label in desc_labels:
        if desc_label in label_to_items:
            positive_items.update(label_to_items[desc_label])
    return list(positive_items)