This is not optimal yet because it still construct a minimal WDBA internally, but it's better than the previous way to call minimize_obligation() since it can avoid constructing the minimized automaton in a few more cases. * spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: Introduce is_obligation(). * bin/ltlfilt.cc: Wire it to --obligation. * spot/twaalgos/minimize.cc: Implement is_wdba_realizable(), needed by the above. * tests/core/obligation.test: Test it. * bin/man/spot-x.x, NEWS: Document it.