'Return relationship of *reltype* to *target_part*, newly added if not
already present in collection.'

def get_or_add(self, reltype, target_part):

    rel = self._get_matching(reltype, target_part)
    if (rel is None):
        rId = self._next_rId
        rel = self.add_relationship(reltype, target_part, rId)
    return rel
