Pick the most readable format available for displaying formulas.

* src/bin/ltlcheck.cc (translator_runner::formula): New function.
(processor::process_formula): Use it.

src/bin/ltlcheck.cc

@@ -270,6 +270,19 @@ namespace
       toclean.push_back(tmpname);
     }
 
+    const std::string& formula() const
+    {
+      // Pick the most readable format we have...
+      if (!string_ltl_spot.val().empty())
+	return string_ltl_spot;
+      if (!string_ltl_spin.val().empty())
+	return string_ltl_spin;
+      if (!string_ltl_lbt.val().empty())
+	return string_ltl_lbt;
+      error(2, 0, "None of the translators need the input formula?");
+      return string_ltl_spot;
+    }
+
     void round_formula(const spot::ltl::formula* f, unsigned serial)
     {
       if (has('f') || has('F'))
@@ -461,21 +474,23 @@ namespace
       (void) linenum;
       static unsigned round = 0;
 
+      size_t m = translators.size();
+
+      std::vector<const spot::tgba*> pos(m);
+      std::vector<const spot::tgba*> neg(m);
+
+      runner.round_formula(f, round);
+      // Call formula() before printing anything else, in case it
+      // complains.
+      std::string fstr = runner.formula();
       if (filename)
 	std::cerr << filename << ":";
       if (linenum)
 	std::cerr << linenum << ":";
       if (filename || linenum)
 	std::cerr << " ";
-      spot::ltl::to_string(f, std::cerr);
-      std::cerr << "\n";
-
-      size_t m = translators.size();
+      std::cerr << fstr << "\n";
 
-      std::vector<const spot::tgba*> pos(m);
-      std::vector<const spot::tgba*> neg(m);
-
-      runner.round_formula(f, round);
       for (size_t n = 0; n < m; ++n)
 	pos[n] = runner.translate(n, 'P');