def _annotation_iter_helper(graph) -> Iterable[str]:
    """Iterate over the annotation keys.

    :param pybel.BELGraph graph: A BEL graph
    """
    return (
        key
        for _, _, data in graph.edges(data=True)
        if ANNOTATIONS in data
        for key in data[ANNOTATIONS]
    )
Iterate over the annotation keys.

    :param pybel.BELGraph graph: A BEL graph