Get rid of Entry.collection

        if key in self.entries:

            report_error(BibliographyDataError('repeated bibliograhpy entry: %s' % key))

            return

        entry.collection = self

        entry.key = self.get_canonical_key(key)

        self.entries[entry.key] = entry

        try:

    The most often used roles are ``'author'`` and ``'editor'``.

    """

    collection = None

    """A reference to the containing :py:class:`.BibliographyData` object. Used to resolve crossrefs."""

    def __init__(self, type_, fields=None, persons=None, collection=None):

    def __init__(self, type_, fields=None, persons=None):

        if fields is None:

            fields = {}

        if persons is None:

        self.persons = OrderedCaseInsensitiveDict(persons)

        self.collection = collection

        # for BibTeX interpreter

        self.vars = {}

            self.type,

            fields=self.fields.lower(),

            persons=self.persons.lower(),

            collection=self.collection,

        )

    def _find_person_field(self, role):