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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 _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]
    )