'Callback for updating.'

def _cb_update(self, a, b, rel):

    if (rel == Conversion.promote):
        self._tm.set_promote(a, b)
    elif (rel == Conversion.safe):
        self._tm.set_safe_convert(a, b)
    elif (rel == Conversion.unsafe):
        self._tm.set_unsafe_convert(a, b)
    else:
        raise AssertionError(rel)
