def METHOD_NAME(self):
    """A circulation event may be collected with either the
    library or the license pool missing, but not both.
    """
    now = utc_now()
    self.la.collect_event(self._default_library, None, "event", now)
    pool = self._licensepool(None)
    self.la.collect_event(None, pool, "event", now)
    with pytest.raises(ValueError) as excinfo:
        self.la.collect_event(None, None, "event", now)
    assert "Either library or license_pool must be provided." in str(excinfo.value)