Add a decompose_strength() function.

* src/twaalgos/strength.cc, src/twaalgos/strength.hh
(decompose_stregth): New function.
* src/bin/autfilt.cc: Add a --decompose-strength option.
* src/bin/man/autfilt.x: Add bibliography.
* src/tests/strength.test: Test it.
* NEWS: Mention it.

src/twaalgos/strength.hh

@@ -68,4 +68,43 @@ namespace spot
   /// will be built otherwise).
   SPOT_API void
   check_strength(const twa_graph_ptr& aut, scc_info* sm = nullptr);
+
+
+  /// \brief Extract a sub-automaton of a given strength
+  ///
+  /// The string \a keep should be a non-empty combination of
+  /// the following letters:
+  /// - 'w': keep only weak SCCs (i.e., SCCs in which all transitions belong
+  ///   to the same acceptance sets) that are not terminal.
+  /// - 't': keep terminal SCCs (i.e., weak SCCs that are complete)
+  /// - 's': keep strong SCCs (i.e., SCCs that are not weak).
+  ///
+  /// This algorithm returns a subautomaton that contains all SCCs of the
+  /// requested strength, plus any upstream SCC (but adjusted not to be
+  /// accepting).
+  ///
+  /** \verbatim
+      @inproceedings{renault.13.tacas,
+        author = {Etienne Renault and Alexandre Duret-Lutz and Fabrice
+		  Kordon and Denis Poitrenaud},
+        title = {Strength-Based Decomposition of the Property {B\"u}chi
+		  Automaton for Faster Model Checking},
+        booktitle = {Proceedings of the 19th International Conference on Tools
+		  and Algorithms for the Construction and Analysis of Systems
+		  (TACAS'13)},
+        editor = {Nir Piterman and Scott A. Smolka},
+	year = {2013},
+	month = mar,
+	pages = {580--593},
+	publisher = {Springer},
+	series = {Lecture Notes in Computer Science},
+	volume = {7795},
+        doi = {10.1007/978-3-642-36742-7_42}
+      }
+      \endverbatim */
+  ///
+  /// \param aut the automaton to decompose
+  /// \param keep a string specifying the strengths to keep: it should
+  SPOT_API twa_graph_ptr
+  decompose_strength(const const_twa_graph_ptr& aut, const char* keep);
 }