* spot/twa/twa.cc (is_empty, intersects): Here.
* spot/twaalgos/sccinfo.cc (check_scc_emptiness): Here.
* spot/twaalgos/genem.cc: Report error if the input is alternating.
* spot/twaalgos/isunamb.cc, spot/twaalgos/sccinfo.hh: Adjust.
* NEWS: Mention the change.