diff --git a/doc/footer.html b/doc/footer.html
index 651057411..f9212b389 100644
--- a/doc/footer.html
+++ b/doc/footer.html
@@ -1,6 +1,6 @@
 <hr size="1">
 Please direct any <b>question</b>, <b>comment</b>, or <b>bug report</b> to the Spot mailing list at <a href="mailto:spot@lrde.epita.fr">spot@lrde.epita.fr</a>.
 <br><address style="align: right;"><small>
-Generated on $datetime for $projectname by <a href="http://www.doxygen.org/index.html"><img src="doxygen.png" alt="doxygen" align="middle" border=0></a> $doxygenversion</small></address>
+Generated on $datetime for $projectname by <a href="http://www.doxygen.org/index.html"><img src="doxygen.png" alt="doxygen" align="middle" border=0 /></a> $doxygenversion</small></address>
 </body>
 </html>
diff --git a/doc/mainpage.dox b/doc/mainpage.dox
index 6c5930f1f..fefd68625 100644
--- a/doc/mainpage.dox
+++ b/doc/mainpage.dox
@@ -22,11 +22,12 @@
 /// \li spot::formula         Base class for an LTL or PSL formula.
 /// \li spot::parse_infix_psl() Parsing a text string into a
 ///                           spot::formula.
+/// \li \ref tl
 /// \li spot::twa             Base class for Transition-based
 ///                           ω-Automata.
-/// \li spot::twa_algorithms  Algorithms on ω-Automata.
 /// \li spot::translator      Convert a spot::formula into a
 ///                           spot::twa.
+/// \li \ref twa_algorithms
 /// \li spot::kripke          Base class for Kripke structures.
 /// \li spot::twa_product     On-the-fly product of two spot::twa.
 /// \li spot::emptiness_check Base class for all emptiness-check algorithms
diff --git a/spot/tl/formula.hh b/spot/tl/formula.hh
index 59cdf4883..53b0a41d1 100644
--- a/spot/tl/formula.hh
+++ b/spot/tl/formula.hh
@@ -37,6 +37,9 @@
 /// \addtogroup tl_rewriting Rewriting Algorithms for Formulas
 /// \ingroup tl
 
+/// \addtogroup tl_hier Algorithms related to the temporal hierarchy
+/// \ingroup tl
+
 /// \addtogroup tl_misc Miscellaneous Algorithms for Formulas
 /// \ingroup tl
 
diff --git a/spot/tl/hierarchy.hh b/spot/tl/hierarchy.hh
index fbf024fd9..c567983ed 100644
--- a/spot/tl/hierarchy.hh
+++ b/spot/tl/hierarchy.hh
@@ -24,7 +24,9 @@
 
 namespace spot
 {
-  /// Enum used to change the behavior of is_persistence() or is_recurrence().
+  /// \ingroup tl_hier
+  /// \brief Enum used to change the behavior of is_persistence() or
+  /// is_recurrence().
   ///
   /// If PR_Auto, both methods will first check the environment variable
   /// <code>SPOT_PR_CHECK</code> to see if one algorithm or the other is wanted
@@ -47,6 +49,7 @@ namespace spot
     via_Rabin,
   };
 
+  /// \ingroup tl_hier
   /// \brief Return true if \a f represents a persistence property.
   ///
   /// \param f the formula to check.
@@ -57,6 +60,7 @@ namespace spot
                  twa_graph_ptr aut = nullptr,
                  prcheck algo = prcheck::Auto);
 
+  /// \ingroup tl_hier
   /// \brief Return true if \a f represents a recurrence property.
   ///
   /// Actually, it calls is_persistence() with the negation of \a f.
@@ -78,6 +82,7 @@ namespace spot
     via_WDBA,
   };
 
+  /// \ingroup tl_hier
   /// \brief Return true if \a f has the recurrence property.
   ///
   /// Actually, it calls is_persistence() with the negation of \a f.
@@ -102,6 +107,7 @@ namespace spot
                 twa_graph_ptr aut = nullptr,
                 ocheck algo = ocheck::Auto);
 
+  /// \ingroup tl_hier
   /// \brief Return the class of \a f in the temporal hierarchy of Manna
   /// and Pnueli (PODC'90).
   ///
@@ -118,6 +124,7 @@ namespace spot
   SPOT_API char mp_class(formula f);
 
 
+  /// \ingroup tl_hier
   /// \brief Return the class of \a f in the temporal hierarchy of Manna
   /// and Pnueli (PODC'90).
   ///
@@ -132,6 +139,7 @@ namespace spot
   /// Any ']' ends the processing of the options.
   SPOT_API std::string mp_class(formula f, const char* opt);
 
+  /// \ingroup tl_hier
   /// \brief Expand a class in the temporal hierarchy of Manna
   /// and Pnueli (PODC'90).
   ///