def __init__(self, id=None, pmid="", pmc="", doi="", url="",
                 authors=None, year="", title="", journal="", abstract=""):
        super(Reference, self).__init__()
        self._linked_items = set()
        self.id = id or str(uuid4())
        self.pmid = pmid
        self.pmc = pmc
        self.doi = doi
        self.url = url
        if authors is None:
            self.authors = []
        else:
            self.authors = authors
        self.year = year
        self.title = title
        self.journal = journal
        self.abstract = abstract