def get_isbn(raw):
    """
    Extract ISBN(s).

    @param raw: json object of a Libris edition
    @type raw: dictionary
    """
    identified_by = raw["mainEntity"].get("identifiedBy")
    if identified_by:
        isbns = [x for x in identified_by
                 if x["@type"].lower() == "isbn"]
        return [x["value"] for x in isbns][0]
    return None