Search trees by `issn`.

        Args:
            issn (str): :attr:`.Tree.issn` property of :class:`.Tree`.

        Returns:
            set: Set of matching :class:`Tree` instances.

def trees_by_issn(self, issn):
        """
        Search trees by `issn`.

        Args:
            issn (str): :attr:`.Tree.issn` property of :class:`.Tree`.

        Returns:
            set: Set of matching :class:`Tree` instances.
        """
        return set(
            self.issn_db.get(issn, OOSet()).keys()
        )