Use 'const formula*' instead of 'formula*' everywhere.

The distinction makes no sense since Spot 0.5, where we switched from
mutable furmulae to immutable formulae.  The difference between
const_visitor and visitor made no sense either.  They have been merged
into one: visitor.

* iface/dve2/dve2check.cc, iface/gspn/ltlgspn.cc,
src/eltlparse/eltlparse.yy, src/eltlparse/public.hh,
src/evtgbatest/ltl2evtgba.cc, src/kripkeparse/kripkeparse.yy,
src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/automatop.cc, src/ltlast/automatop.hh, src/ltlast/binop.cc,
src/ltlast/binop.hh, src/ltlast/bunop.cc, src/ltlast/bunop.hh,
src/ltlast/constant.cc, src/ltlast/constant.hh, src/ltlast/formula.cc,
src/ltlast/formula.hh, src/ltlast/formula_tree.cc,
src/ltlast/formula_tree.hh, src/ltlast/multop.cc,
src/ltlast/multop.hh, src/ltlast/predecl.hh, src/ltlast/refformula.cc,
src/ltlast/refformula.hh, src/ltlast/unop.cc, src/ltlast/unop.hh,
src/ltlast/visitor.hh, src/ltlenv/declenv.cc, src/ltlenv/declenv.hh,
src/ltlenv/defaultenv.cc, src/ltlenv/defaultenv.hh,
src/ltlenv/environment.hh, src/ltlparse/ltlfile.cc,
src/ltlparse/ltlfile.hh, src/ltlparse/ltlparse.yy,
src/ltlparse/public.hh, src/ltltest/consterm.cc,
src/ltltest/equals.cc, src/ltltest/genltl.cc, src/ltltest/kind.cc,
src/ltltest/length.cc, src/ltltest/randltl.cc, src/ltltest/readltl.cc,
src/ltltest/reduc.cc, src/ltltest/syntimpl.cc,
src/ltltest/tostring.cc, src/ltlvisit/apcollect.cc,
src/ltlvisit/apcollect.hh, src/ltlvisit/clone.cc,
src/ltlvisit/clone.hh, src/ltlvisit/contain.cc,
src/ltlvisit/contain.hh, src/ltlvisit/dotty.cc,
src/ltlvisit/length.cc, src/ltlvisit/lunabbrev.cc,
src/ltlvisit/lunabbrev.hh, src/ltlvisit/mark.cc, src/ltlvisit/mark.hh,
src/ltlvisit/nenoform.cc, src/ltlvisit/nenoform.hh,
src/ltlvisit/postfix.cc, src/ltlvisit/postfix.hh,
src/ltlvisit/randomltl.cc, src/ltlvisit/randomltl.hh,
src/ltlvisit/reduce.cc, src/ltlvisit/reduce.hh,
src/ltlvisit/simpfg.cc, src/ltlvisit/simpfg.hh,
src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh,
src/ltlvisit/snf.cc, src/ltlvisit/snf.hh, src/ltlvisit/tostring.cc,
src/ltlvisit/tunabbrev.cc, src/ltlvisit/tunabbrev.hh,
src/ltlvisit/wmunabbrev.cc, src/ltlvisit/wmunabbrev.hh,
src/neverparse/neverclaimparse.yy, src/sabatest/sabacomplementtgba.cc,
src/tgba/bdddict.cc, src/tgba/formula2bdd.cc, src/tgba/taatgba.cc,
src/tgba/taatgba.hh, src/tgbaalgos/eltl2tgba_lacim.cc,
src/tgbaalgos/ltl2taa.cc, src/tgbaalgos/ltl2tgba_fm.cc,
src/tgbaalgos/ltl2tgba_lacim.cc, src/tgbaalgos/minimize.cc,
src/tgbaalgos/randomgraph.cc, src/tgbaparse/tgbaparse.yy,
src/tgbatest/complementation.cc, src/tgbatest/ltl2tgba.cc,
src/tgbatest/ltlprod.cc, src/tgbatest/mixprod.cc,
src/tgbatest/randtgba.cc: Massive adjustment!
* src/tgbatest/reductgba.cc: Delete.

src/ltlparse/ltlfile.cc

@@ -1,4 +1,4 @@
-// Copyright (C) 2010 Laboratoire de Recherche et Développement
+// Copyright (C) 2010, 2012 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -46,7 +46,7 @@ namespace spot
 	}
     }
 
-    formula* ltl_file::next()
+    const formula* ltl_file::next()
     {
       if (!in.good())
 	return 0;
@@ -60,7 +60,7 @@ namespace spot
       while (input == "");
 
       spot::ltl::parse_error_list pel;
-      formula* f = parse(input, pel);
+      const formula* f = parse(input, pel);
       int ret = spot::ltl::format_parse_errors(std::cerr, input, pel);
       if (ret)
 	exit(ret);

src/ltlparse/ltlfile.hh

@@ -1,4 +1,4 @@
-// Copyright (C) 2010 Laboratoire de Recherche et Développement
+// Copyright (C) 2010, 2012 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -37,7 +37,7 @@ namespace spot
       ltl_file(const std::string& filename);
       ltl_file(const char* filename);
       /// Return the next parsed LTL formula, and 0 at end of file.
-      formula* next();
+      const formula* next();
     private:
       std::ifstream in;
     };

src/ltlparse/ltlparse.yy

@@ -41,11 +41,11 @@
 
 %parse-param {spot::ltl::parse_error_list &error_list}
 %parse-param {spot::ltl::environment &parse_environment}
-%parse-param {spot::ltl::formula* &result}
+%parse-param {const spot::ltl::formula* &result}
 %union
 {
   std::string* str;
-  spot::ltl::formula* ltl;
+  const spot::ltl::formula* ltl;
   unsigned num;
   minmax_t minmax;
 }
@@ -671,13 +671,13 @@ namespace spot
 {
   namespace ltl
   {
-    formula*
+    const formula*
     parse(const std::string& ltl_string,
 	  parse_error_list& error_list,
 	  environment& env,
 	  bool debug)
     {
-      formula* result = 0;
+      const formula* result = 0;
       flex_set_buffer(ltl_string.c_str(),
 		      ltlyy::parser::token::START_LTL);
       ltlyy::parser parser(error_list, env, result);
@@ -686,13 +686,13 @@ namespace spot
       return result;
     }
 
-    formula*
+    const formula*
     parse_sere(const std::string& sere_string,
 		 parse_error_list& error_list,
 		 environment& env,
 		 bool debug)
     {
-      formula* result = 0;
+      const formula* result = 0;
       flex_set_buffer(sere_string.c_str(),
 		      ltlyy::parser::token::START_SERE);
       ltlyy::parser parser(error_list, env, result);

src/ltlparse/public.hh

@@ -67,10 +67,10 @@ namespace spot
     /// was parsed succesfully, check \a error_list for emptiness.
     ///
     /// \warning This function is not reentrant.
-    formula* parse(const std::string& ltl_string,
-		   parse_error_list& error_list,
-		   environment& env = default_environment::instance(),
-		   bool debug = false);
+    const formula* parse(const std::string& ltl_string,
+			 parse_error_list& error_list,
+			 environment& env = default_environment::instance(),
+			 bool debug = false);
 
     /// \brief Build a formula from a string representing a SERE.
     /// \param sere_string The string to parse.
@@ -87,10 +87,11 @@ namespace spot
     /// was parsed succesfully, check \a error_list for emptiness.
     ///
     /// \warning This function is not reentrant.
-    formula* parse_sere(const std::string& sere_string,
-			parse_error_list& error_list,
-			environment& env = default_environment::instance(),
-			bool debug = false);
+    const formula* parse_sere(const std::string& sere_string,
+			      parse_error_list& error_list,
+			      environment& env =
+			      default_environment::instance(),
+			      bool debug = false);
 
     /// \brief Format diagnostics produced by spot::ltl::parse
     ///        or spot::ltl::ratexp