def obo(self):
        """str: the synonym serialized in obo format.
        """
        return 'synonym: "{}" {} [{}]'.format(
            self.desc,
            ' '.join([self.scope, self.syn_type.name])\
                if self.syn_type else self.scope,
            ', '.join(self.xref)
        )
str: the synonym serialized in obo format.