ltlcross: diagnose complementations requiring too many colors

Fixes #411 reported by Frantiček Blahoudek.

* bin/ltlcross.cc: Catch the issue.
* tests/core/ltlcross6.test: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention it.

NEWS

@@ -66,6 +66,11 @@ New in spot 2.9.0.dev (not yet released)
     issues in intersecting_run(), exclusive_run(),
     intersecting_word(), exclusive_word(), which all call reduce().
 
+  - ltlcross used to crash with "Too many acceptance sets used.  The
+    limit is 32." when complementing an automaton would require more
+    acceptance sets than supported by Spot.  This is now diagnosed
+    with --verbose, but does not prevent ltlcross from continuing.
+
 New in spot 2.9 (2020-04-30)
 
   Command-line tools:

bin/ltlcross.cc

@@ -1336,25 +1336,37 @@ namespace
                           && !spot::is_universal(from[i]))
                         missing_complement = true;
                       else
-                        to[i] = spot::complement(from[i], aborter);
-                      if (verbose)
-                        {
-                          if (to[i])
-                            {
-                              std::cerr << "\t(";
-                              printsize(from[i]);
-                              std::cerr << ") -> (";
-                              printsize(to[i]);
-                              std::cerr << ")\tComp(" << prefix << i << ")\n";
-                            }
-                          else
-                            {
-                              std::cerr << "\tnot complemented";
-                              if (aborter)
-                                aborter->print_reason(std::cerr << " (") << ')';
-                              std::cerr << '\n';
-                            }
-                        }
+                        try
+                          {
+                            to[i] = spot::complement(from[i], aborter);
+                            if (verbose)
+                              {
+                                if (to[i])
+                                  {
+                                    std::cerr << "\t(";
+                                    printsize(from[i]);
+                                    std::cerr << ") -> (";
+                                    printsize(to[i]);
+                                    std::cerr << ")\tComp(" << prefix
+                                              << i << ")\n";
+                                  }
+                                else
+                                  {
+                                    std::cerr << "\tnot complemented";
+                                    if (aborter)
+                                      aborter->print_reason(std::cerr
+                                                            << " (") << ')';
+                                    std::cerr << '\n';
+                                  }
+                              }
+                          }
+                        catch (const std::runtime_error& re)
+                          {
+                            missing_complement = true;
+                            if (verbose)
+                              std::cerr << "\tnot complemented ("
+                                        << re.what() << ")\n";
+                          }
                     }
                 };
               missing_complement = false;

tests/Makefile.am

@@ -310,6 +310,7 @@ TESTS_twa = \
   core/ltlcross.test \
   core/spotlbtt2.test \
   core/ltlcross2.test \
+  core/ltlcross6.test \
   core/autcross.test \
   core/autcross2.test \
   core/autcross3.test \