org: examples with alternating automata

* doc/org/tut23.org, doc/org/tut24.org, doc/org/tut31.org: New files.
* doc/Makefile.am, doc/org/tut.org: Add them.
* doc/org/hoa.org, doc/org/concepts.org: Adjust for alternation support.
* NEWS: Add links.

doc/org/concepts.org

@@ -554,6 +554,75 @@ $txt
 #+RESULTS:
 [[file:concept-twa1.png]]
 
+
+* Alternating ω-automata
+
+Alternating ω-automata are ω-automata in which the destination of an
+edge can be a group of states.  If an edge has more than one
+destination, it is called a /universal edge/, and its destinations are
+referred to as its /universal destinations/.
+
+When an alternating automaton evaluates a word, following a universal
+edge will have the same effect as forking the automaton to evaluate
+the rest of the word simultaneously from each universal destination.
+A run of an alternating automaton can therefore be pictured as a tree.
+The tree is accepting if all its branches satisfy the acceptance condition.
+(See the [[http://adl.github.io/hoaf/][Hanoi Omega-Automa format]] for more precise semantics.)
+
+For instance the following alternating co-Büchi ω-automaton was
+generated by [[https://sourceforge.net/projects/ltl3ba/][=ltl3ba 1.1.3=]] for the LTL formula =GF(a <-> Xb)=.
+
+#+NAME: concepts-alt
+#+BEGIN_SRC sh :results verbatim :exports none
+autfilt -d.ba <<EOF
+HOA: v1
+States: 5
+Start: 0
+acc-name: co-Buchi
+Acceptance: 1 Fin(0)
+AP: 2 "a" "b"
+properties: trans-labels explicit-labels state-acc univ-branch very-weak
+--BODY--
+State: 0 "GF(a <-> Xb)"
+ [(!0)] 3&0
+ [(0)] 2&0
+ [t] 1&0
+State: 1 "F(a <-> Xb)" {0}
+ [(!0)] 3
+ [(0)] 2
+ [t] 1
+State: 2 "b"
+ [(1)] 4
+State: 3 "!b"
+ [(!1)] 4
+State: 4 "t"
+ [t] 4
+--END--
+EOF
+#+END_SRC
+
+#+BEGIN_SRC dot :file concepts-alt.png :cmdline -Tpng :var txt=concepts-alt :exports results
+$txt
+#+END_SRC
+
+#+RESULTS:
+[[file:concepts-alt.png]]
+
+In this picture, the universal edges appear as arrows with a white
+tips going to a small dot, from which additional arrows connect to the
+universal destinations.  Here the three universal edges all leave the
+initial state, and connect to two universal destinations.  Note that
+non-determinism is allowed between universal edges, for instance upon
+reading a word starting with "=a=", this automaton should
+non-deterministically decide to read the rest of the word from states
+=GF(a<->Xb)= and =F(a<->Xb)= (when taking the universal transition
+labeled by =1=) or from states =GF(a<->Xb)= and =b= (when taking the
+universal transition labeled by =a=).
+
+Alternation support in Spot is currently experimental, please report
+any issue.  The only supported file format able to represent
+alternating automata is the [[#hoa][HOA format, introduced below]].
+
 * Never claims
    :PROPERTIES:
    :CUSTOM_ID: neverclaim
@@ -770,7 +839,7 @@ $txt
 
 The [[http://adl.github.io/hoaf/][HOA format]] inherits several features from the [[:dstar][DSTAR format]], but
 extends it in many ways, including support for non-deterministic
-automata and for arbitrary acceptance conditions.
+automata, alternating automata, and for arbitrary acceptance conditions.
 
 #+BEGIN_SRC sh :results verbatim :exports results
 ltldo ltl2dstar -f 'FGp0 | GFp1' --name=%f
@@ -826,13 +895,10 @@ $txt
 Since this file format is the only one able to represent the range of
 ω-automata supported by Spot, and it its default output format.
 
-However note that Spot does not (yet?) support all automata that can
-be expressed using the HOA format.  For instance it has no support for
-alternating automata, or for the =Alphabet:=-based automata introduced
-in v1.1 of HOA (only =AP:=-based automata are supported).
-
-The present support for the HOA format in Spot, is discussed on [[file:hoa.org][a
-separate page]].
+However note that Spot does not support all automata that can be
+expressed using the HOA format.  The present support for the HOA
+format in Spot, is discussed on [[file:hoa.org][a separate page]], with a section
+dedicated to the [[file:hoa.org::#restrictions][restrictions]].
 
 * Linear-time Temporal Logic (LTL)
    :PROPERTIES:
@@ -975,7 +1041,6 @@ program.  Blue boxes are Python-related.
    :CUSTOM_ID: property-flags
    :END:
 
-
 The automaton class used by Spot to represent ω-Automata is called
 =twa= (because we use TωA as a short for Transition-based
 ω-Automaton).  As its names implies, the =twa= class supports only