def bind(self, existing, imported):
        [match1] = [m for m in self.matches if m[0] is existing]
        [match2] = [m for m in self.matches if m[1] is imported]
        assert match1[1] is None
        assert match2[0] is None
        match1[1] = match2[1]
        self.matches.remove(match2)