def get_nodes_labels(properties):
    """Returns the node labels contained in given property graphs.

    :param properties: An iterable of TimedPropertyGraph objects.

    :return: A set containing all node labels used in given sequence of property graphs.
    """
    labels = set()
    for p in properties:
        for n in p.graph.nodes:
            labels.add(p.get_node_label(n))
    return labels