new conversion for Streett->GBA

* src/twaalgos/totgba.hh, src/twaalgos/totgba.cc: Implement
the new function.
* NEWS: Mention this new function.
* src/bin/man/spot-x.x: Document SPOT_STREETT_CONV_MIN.
* src/tests/ltl2dstar4.test: Add tests.
* src/tests/Makefile.am: Add it.
* src/bin/autfilt.cc: Do do call remove_fin explicitely
when --tgba is used, let the postprocessor do it.
* src/twa/acc.hh: Add shift operators for acceptance marks.
* src/twaalgos/remfin.cc: Use the new algorithm.
* src/twaalgos/sccinfo.cc, src/twaalgos/sccinfo.hh: Add
a new method to supply the acceptance sets visited by an SCC.

NEWS

@@ -13,6 +13,24 @@ New in spot 1.99.1a (not yet released)
     translation of one PSL formula and its now takes) now takes only
     75s.
 
+  * streett_to_generalized_buchi() is a new function that implements
+    what its name suggests, with some SCC-based optimizations over the
+    naive definition.  It is used by the to_generalized_buchi() and
+    remove_fin() function when the input automaton is a Streett
+    automaton with more than 3 pairs (this threeshold can be changed
+    via the SPOT_STREETT_CONV_MIN environment variable -- see the
+    spot-x(7) man page for details).
+
+    This is mainly useful to ltlcross, which has to get rid of "Fin"
+    acceptance to perform its checks.  As an example, the Streett
+    automaton generatated by ltl2dstar (configured with ltl2tgba) for
+    the formula
+      !((GFa -> GFb) & (GFc -> GFd))
+    has 4307 states and 14 pairs.  The new algorithm can translate it
+    into a TGBA with 9754 states and 7 acceptance sets, while the
+    default approch used for converting any acceptance to TGBA would
+    produce 250967 states and 7 acceptance sets.
+
   * Bugs fixed:
     - p[+][:*2] was not detected as belonging to siPSL
     - scc_filter() would incorrectly remove Fin marks from