introduce is_obligation(f)

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.

bin/ltlfilt.cc

@@ -732,7 +732,7 @@ namespace
               // Because this is costly, we compute it later, so that we don't
               // have to compute it on formulas that have been discarded for
               // other reasons.
-              if (obligation)
+              if (obligation && (safety || guarantee))
                 {
                   if (!aut)
                     aut = ltl_to_tgba_fm(f, simpl.get_dict(), true);
@@ -752,10 +752,10 @@ namespace
                       matched &= !safety || is_safety_automaton(min, &si);
                     }
                 }
-
+              else if (obligation) // just obligation, not safety or guarantee
+                  matched &= is_obligation(f, aut);
               else if (persistence)
                 matched &= spot::is_persistence(f);
-
               else if (recurrence)
                 matched &= spot::is_recurrence(f, aut);
             }

bin/man/spot-x.x

@@ -99,6 +99,23 @@ in third-party tools that output incorrect HOA (e.g., declaring
 the automaton with property "univ-branch" when no universal branching
 is actually used)
 
+.TP
+\fBSPOT_O_CHECK\fR
+Specifies the default algorithm that should be used
+by the \f(CWis_obligation()\fR function.  The value should
+be one of the following:
+.RS
+.IP 1
+Make sure that the formula and its negation are
+realizable by non-deterministic co-Büchi automata.
+.IP 2
+Make sure that the formula and its negation are
+realizable by deterministic Büchi automata.
+.IP 3
+Make sure that the formula is realizable
+by a weak and deterministic Büchi automata.
+.RE
+
 .TP
 \fBSPOT_OOM_ABORT\fR
 If this variable is set, Out-Of-Memory errors will \f(CWabort()\fR the