add support for the "terminal" property

* src/twa/twa.hh: Store the terminal property.
* src/twaalgos/hoa.cc, src/parseaut/parseaut.yy: Add I/O for "terminal".
* src/twaalgos/ltl2tgba_fm.cc, src/twaalgos/minimize.cc: Set terminal
as apropriate.
* src/twaalgos/safety.cc: Use it.
* doc/org/tut21.org, doc/org/hoa.org, NEWS: Document it.
* src/tests/complement.test, src/tests/monitor.test,
wrap/python/tests/automata-io.ipynb: Adjust.

src/twaalgos/hoa.cc

@@ -425,7 +425,9 @@ namespace spot
       prop(" stutter-invariant");
     if (aut->prop_stutter_sensitive())
       prop(" stutter-sensitive");
-    if (aut->prop_weak())
+    if (aut->prop_terminal())
+      prop(" terminal");
+    if (aut->prop_weak() && (verbose || !aut->prop_terminal()))
       prop(" weak");
     if (aut->prop_inherently_weak() && (verbose || !aut->prop_weak()))
       prop(" inherently-weak");

src/twaalgos/ltl2tgba_fm.cc

@@ -1972,6 +1972,8 @@ namespace spot
       }
     bdd all_events = observable_events | unobservable_events;
 
+    auto orig_f = f2;
+
     // This is in case the initial state is equivalent to true...
     if (symb_merge)
       f2 = fc.canonize(f2);
@@ -2172,9 +2174,6 @@ namespace spot
 	}
       }
 
-    // Set the following to true to preserve state names.
-    a->release_formula_namer(namer, false);
-
     auto& acc = a->acc();
     unsigned ns = a->num_states();
     for (unsigned s = 0; s < ns; ++s)
@@ -2185,10 +2184,18 @@ namespace spot
 
     a->prop_inherently_weak(f2.is_syntactic_persistence());
     a->prop_stutter_invariant(f2.is_syntactic_stutter_invariant());
+    if (orig_f.is_syntactic_guarantee())
+      {
+	a->prop_terminal(true);
+	assert(a->num_sets() <= 1);
+      }
 
     // Currently the unambiguous option work only with LTL.
     a->prop_unambiguous(f2.is_ltl_formula() && unambiguous);
 
+    // Set the following to true to preserve state names.
+    a->release_formula_namer(namer, false);
+
     if (!simplifier)
       // This should not be deleted before we have registered all propositions.
       delete s;

src/twaalgos/minimize.cc

@@ -584,6 +584,16 @@ namespace spot
     res->prop_copy(a, { false, false, false, true });
     res->prop_deterministic(true);
     res->prop_weak(true);
+    // If the input was terminal, then the output is also terminal.
+    // FIXME:
+    // (1) We should have a specialized version of this function for
+    // the case where the input is terminal.  See issue #120.
+    // (2) It would be nice to have a more precise detection of
+    // terminal automata in the output.  Calling
+    // is_guarantee_automaton() seems overkill here.  But maybe we can
+    // add a quick check inside minimize_dfa.
+    if (a->prop_terminal())
+      res->prop_terminal(true);
     return res;
   }
 

src/twaalgos/safety.cc

@@ -27,6 +27,8 @@ namespace spot
   is_guarantee_automaton(const const_twa_graph_ptr& aut,
 			 scc_info* si)
   {
+    if (aut->prop_terminal())
+      return true;
     // Create an scc_info if the user did not give one to us.
     bool need_si = !si;
     if (need_si)