def parse(self, limit=None):
        """
        Override Source.parse()
        Args:
            :param limit (int, optional) limit the number of rows processed
        Returns:
            :return None
        """
        if limit is not None:
            LOG.info("Only parsing first %d rows", limit)

        rgd_file = '/'.join(
            (self.rawdir, self.files['rat_gene2mammalian_phenotype']['file']))
        # ontobio gafparser implemented here
        p = GafParser()
        assocs = p.parse(open(rgd_file, "r"))

        for i, assoc in enumerate(assocs):
            if 'relation' in assoc.keys():
                self.make_association(assoc)
            if limit is not None and i > limit:
                break
        return
Override Source.parse()
        Args:
            :param limit (int, optional) limit the number of rows processed
        Returns:
            :return None