The algorithm had two problems: it was removing only useless
destination from universal destination (instead of removing the entire
edge), and it was not properly iterating over the entire reachable
automaton.
* spot/twa/twagraph.cc: Fix it.
* spot/twa/twagraph.hh: Adjust documentation.
* tests/core/alternating.test: Add more tests.
* tests/python/twagraph.py: Adjust.
* NEWS: Mention the bug.