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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 iterate_pubmed_identifiers(graph) -> Iterable[str]: """Iterate over all PubMed identifiers in a graph. :param pybel.BELGraph graph: A BEL graph :return: An iterator over the PubMed identifiers in the graph """

    return (
        data[CITATION][CITATION_REFERENCE].strip()
        for _, _, data in graph.edges(data=True)
        if has_pubmed(data)
    )