#+TITLE: =ltl2tgba=
#+EMAIL spot@lrde.epita.fr
#+OPTIONS: H:2 num:nil toc:t
#+LINK_UP: file:tools.html

This tool translates LTL or PSL formulas into two kinds of Büchi
automata, or to monitors.  The default is to output Transition-based
Generalized Büchi Automata (hereinafter abbreviated TGBA), but more
traditional Büchi automata (BA) may be requested using the =-B=
option.

* TGBA and BA

Formulas to translate may be specified using [[file:ioltl.org][common input options for
LTL/PSL formulas]].

#+BEGIN_SRC sh :results verbatim :exports both
ltl2tgba -f 'Fa & GFb'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1"]
  1 -> 1 [label="!a\n"]
  1 -> 2 [label="a\n"]
  2 [label="2"]
  2 -> 2 [label="b\n{Acc[b]}"]
  2 -> 2 [label="!b\n"]
}
#+end_example

Actually, because =ltl2tgba= is often used with a single formula
passed on the command line, the =-f= option can be omitted and any
command-line parameter that is not the argument of some option will be
assumed to be a formula to translate (this differs from [[file:ltlfilt.org][=ltlfilt=]],
where such parameters are assumed to be filenames).

The default output format, as shown above, is [[http://http://www.graphviz.org/][GraphViz]]'s format.  This
can converted into a picture, or into vectorial format using =dot= or
=dotty=.  Typically, you could get a =pdf= of this TGBA using
#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba "Fa & GFb" | dot -Tpdf > tgba.pdf
#+END_SRC
#+RESULTS:

The result would look like this:
#+NAME: dotex
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba "Fa & GFb" | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: dotex
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1"]
  1 -> 2 [label="a\\n"]
  1 -> 1 [label="!a\\n"]
  2 [label="2"]
  2 -> 2 [label="b\\n{Acc[b]}"]
  2 -> 2 [label="!b\\n"]
}
#+end_example

#+BEGIN_SRC dot :file dotex.png :cmdline -Tpng :var txt=dotex :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:dotex.png]]

The string between braces, =Acc[b]=, represents an acceptance set (its
actual name is not really important): any transition labeled by
=Acc[b]= belongs to the =Acc[b]= acceptance set.  You may have many
transitions in the same acceptance set, and a transition may also
belong to multiple acceptance sets.  An infinite path through this
automaton is accepting iff it visit each acceptance set infinitely
often.  Therefore, in the above example, any accepted path will
/necessarily/ leave the initial state after a finite amount of steps,
and then it will verify the property =b= infinitely often.  It is also
possible that an automaton do not use any acceptance set at all, in
which any run is accepting.

Here is a TGBA with multiple acceptance sets (we omit the call to
=dot= to render the output of =ltl2tgba= from now on):

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba 'GFa & GFb'
#+END_SRC
#+RESULTS:
: digraph G {
:   0 [label="", style=invis, height=0]
:   0 -> 1
:   1 [label="1"]
:   1 -> 1 [label="a & b\n{Acc[b], Acc[a]}"]
:   1 -> 1 [label="b & !a\n{Acc[b]}"]
:   1 -> 1 [label="a & !b\n{Acc[a]}"]
:   1 -> 1 [label="!b & !a\n"]
: }

#+NAME: dotex2
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba "GFa & GFb" | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: dotex2
: digraph G {
:   0 [label="", style=invis, height=0]
:   0 -> 1
:   1 [label="1"]
:   1 -> 1 [label="a & b\\n{Acc[b], Acc[a]}"]
:   1 -> 1 [label="b & !a\\n{Acc[b]}"]
:   1 -> 1 [label="a & !b\\n{Acc[a]}"]
:   1 -> 1 [label="!b & !a\\n"]
: }

#+BEGIN_SRC dot :file dotex2.png :cmdline -Tpng :var txt=dotex2 :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:dotex2.png]]

The above TGBA has two acceptance sets: =Acc[a]= and =Acc[b]=.
The position of these acceptance sets ensures that =a= and =b= atomic
proposition must be true infinitely often.

A Büchi automaton for the previous formula can be obtained with the
=-B= option:

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba -B 'GFa & GFb'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="0", peripheries=2]
  1 -> 1 [label="a & b\n{Acc[1]}"]
  1 -> 2 [label="b & !a\n{Acc[1]}"]
  1 -> 3 [label="!b\n{Acc[1]}"]
  2 [label="1"]
  2 -> 1 [label="a\n"]
  2 -> 2 [label="!a\n"]
  3 [label="2"]
  3 -> 1 [label="a & b\n"]
  3 -> 2 [label="b & !a\n"]
  3 -> 3 [label="!b\n"]
}
#+end_example

#+NAME: dotex2ba
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba -B 'GFa & GFb' | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: dotex2ba
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="0", peripheries=2]
  1 -> 1 [label="a & b\\n{Acc[1]}"]
  1 -> 2 [label="b & !a\\n{Acc[1]}"]
  1 -> 3 [label="!b\\n{Acc[1]}"]
  2 [label="1"]
  2 -> 1 [label="a\\n"]
  2 -> 2 [label="!a\\n"]
  3 [label="2"]
  3 -> 1 [label="a & b\\n"]
  3 -> 2 [label="b & !a\\n"]
  3 -> 3 [label="!b\\n"]
}
#+end_example

#+BEGIN_SRC dot :file dotex2ba.png :cmdline -Tpng :var txt=dotex2ba :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:dotex2ba.png]]

Although accepting states in the Büchi automaton are pictured with
double-lines, internally this automaton is still handled as a TGBA
with a single acceptance set =Acc[1]= such that the transitions
leaving the state are either all accepting, or all non-accepting.
This is the reason why the =Acc[1]= sets are still shown in the
output: it shows that a Büchi automaton is (a special case of) a TGBA.

Various options controls the output format of =ltl2tgba=:

#+BEGIN_SRC sh :results verbatim :exports results
ltl2tgba --help | sed -n '/Output format:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
:   -8, --utf8                 enable UTF-8 characters in output (ignored with
:                              --lbtt or --spin)
:       --dot                  GraphViz's format (default)
:       --lbtt                 LBTT's format
:   -s, --spin                 Spin neverclaim (implies --ba)
:       --spot                 SPOT's format
:       --stats=FORMAT         output statistics about the automaton


Option =-8= can be used to improve the readability of the output
if your system can display UTF-8 correctly.

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba -B8 'GFa & GFb'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="0", peripheries=2]
  1 -> 1 [label="a∧b\n{Acc[1]}"]
  1 -> 2 [label="b∧a̅\n{Acc[1]}"]
  1 -> 3 [label="b̅\n{Acc[1]}"]
  2 [label="1"]
  2 -> 1 [label="a\n"]
  2 -> 2 [label="a̅\n"]
  3 [label="2"]
  3 -> 1 [label="a∧b\n"]
  3 -> 2 [label="b∧a̅\n"]
  3 -> 3 [label="b̅\n"]
}
#+end_example

#+NAME: dotex2ba8
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba -B8 "GFa & GFb" | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: dotex2ba8
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="0", peripheries=2]
  1 -> 1 [label="a∧b\\n{Acc[1]}"]
  1 -> 2 [label="b∧a̅\\n{Acc[1]}"]
  1 -> 3 [label="b̅\\n{Acc[1]}"]
  2 [label="1"]
  2 -> 1 [label="a\\n"]
  2 -> 2 [label="a̅\\n"]
  3 [label="2"]
  3 -> 1 [label="a∧b\\n"]
  3 -> 2 [label="b∧a̅\\n"]
  3 -> 3 [label="b̅\\n"]
}
#+end_example

#+BEGIN_SRC dot :file dotex2ba8.png :cmdline -Tpng :var txt=dotex2ba8 :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:dotex2ba8.png]]

* Spin output

Using the =--spin= or =-s= option, =ltl2tgba= will produce a Büchi automaton
(the =-B= option is implied) as a never claim that can be fed to Spin.
=ltl2tgba -s= is therefore a drop-in replacement for =spin -f=.


#+BEGIN_SRC sh :results verbatim :exports both
ltl2tgba -s 'GFa & GFb'
#+END_SRC
#+RESULTS:
#+begin_example
never { /* G(Fa & Fb) */
accept_init:
  if
  :: ((a) && (b)) -> goto accept_init
  :: ((b) && (!((a)))) -> goto T0_S2
  :: ((!((b)))) -> goto T0_S3
  fi;
T0_S2:
  if
  :: ((a)) -> goto accept_init
  :: ((!((a)))) -> goto T0_S2
  fi;
T0_S3:
  if
  :: ((a) && (b)) -> goto accept_init
  :: ((b) && (!((a)))) -> goto T0_S2
  :: ((!((b)))) -> goto T0_S3
  fi;
}
#+end_example

Since Spin 6 extended its syntax to support arbitrary atomic
propositions, you may also need put the parser in =--lenient= mode to
support these:

#+BEGIN_SRC sh :results verbatim :exports both
ltl2tgba -s --lenient '(a < b) U (process[2]@ok)'
#+END_SRC
#+RESULTS:
: never { /* "a < b" U "process[2]@ok" */
: T0_init:
:   if
:   :: ((process[2]@ok)) -> goto accept_all
:   :: ((a < b) && (!(process[2]@ok))) -> goto T0_init
:   fi;
: accept_all:
:   skip
: }


* Do you favor deterministic or small automata?

The translation procedure can be controled by a few switches.  A first
set of options specifies the intent of the translation: whenever
possible, would you prefer a small automaton or a deterministic
automaton?

#+BEGIN_SRC sh :results verbatim :exports results
ltl2tgba --help | sed -n '/Translation intent:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
:   -a, --any                  no preference
:   -D, --deterministic        prefer deterministic automata
:       --small                prefer small automata (default)

The =--any= option tells the translator that it should not target any
particular form of result: any automaton denoting the given formula is
OK.  This effectively disables post-processings and speeds up the
translation.

With the =-D= option, the translator will /attempt/ to produce a
deterministic automaton, even if this requires a lot of states.  =ltl2tgba=
knows how to produce the minimal deterministic Büchi automaton for
any obligation property (this includes safety properties).

With the =--small= option (the default), the translator will not
produce a deterministic automaton when it knows how to build smaller
automaton.

An example formula where the difference between =-D= and =--small= is
flagrant is =Ga|Gb|Gc=:

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba 'Ga|Gb|Gc'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1"]
  1 -> 2 [label="b\n"]
  1 -> 3 [label="c\n"]
  1 -> 4 [label="a\n"]
  2 [label="2"]
  2 -> 2 [label="b\n"]
  3 [label="3"]
  3 -> 3 [label="c\n"]
  4 [label="4"]
  4 -> 4 [label="a\n"]
}
#+end_example

#+NAME: gagbgc1
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba "Ga|Gb|Gc" | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: gagbgc1
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1"]
  1 -> 2 [label="c\\n"]
  1 -> 3 [label="b\\n"]
  1 -> 4 [label="a\\n"]
  2 [label="2"]
  2 -> 2 [label="c\\n"]
  3 [label="3"]
  3 -> 3 [label="b\\n"]
  4 [label="4"]
  4 -> 4 [label="a\\n"]
}
#+end_example

#+BEGIN_SRC dot :file gagbgc1.png :cmdline -Tpng :var txt=gagbgc1 :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:gagbgc1.png]]

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba -D 'Ga|Gb|Gc'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="6"]
  1 -> 1 [label="a & b & c\n{Acc[1]}"]
  1 -> 2 [label="b & c & !a\n{Acc[1]}"]
  1 -> 3 [label="a & c & !b\n{Acc[1]}"]
  1 -> 4 [label="c & !a & !b\n{Acc[1]}"]
  1 -> 5 [label="a & b & !c\n{Acc[1]}"]
  1 -> 6 [label="b & !a & !c\n{Acc[1]}"]
  1 -> 7 [label="a & !b & !c\n{Acc[1]}"]
  2 [label="2"]
  2 -> 2 [label="b & c\n{Acc[1]}"]
  2 -> 4 [label="c & !b\n{Acc[1]}"]
  2 -> 6 [label="b & !c\n{Acc[1]}"]
  3 [label="4"]
  3 -> 3 [label="a & c\n{Acc[1]}"]
  3 -> 4 [label="c & !a\n{Acc[1]}"]
  3 -> 7 [label="a & !c\n{Acc[1]}"]
  4 [label="1"]
  4 -> 4 [label="c\n{Acc[1]}"]
  5 [label="5"]
  5 -> 5 [label="a & b\n{Acc[1]}"]
  5 -> 6 [label="b & !a\n{Acc[1]}"]
  5 -> 7 [label="a & !b\n{Acc[1]}"]
  6 [label="3"]
  6 -> 6 [label="b\n{Acc[1]}"]
  7 [label="0"]
  7 -> 7 [label="a\n{Acc[1]}"]
}
#+end_example

#+NAME: gagbgc2
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba -D 'Ga|Gb|Gc' | sed 's/\\/\\\\/'
#+END_SRC
#+RESULTS: gagbgc2
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="6"]
  1 -> 1 [label="a & b & c\\n{Acc[1]}"]
  1 -> 2 [label="b & c & !a\\n{Acc[1]}"]
  1 -> 3 [label="a & c & !b\\n{Acc[1]}"]
  1 -> 4 [label="c & !a & !b\\n{Acc[1]}"]
  1 -> 5 [label="a & b & !c\\n{Acc[1]}"]
  1 -> 6 [label="b & !a & !c\\n{Acc[1]}"]
  1 -> 7 [label="a & !b & !c\\n{Acc[1]}"]
  2 [label="1"]
  2 -> 2 [label="b & c\\n{Acc[1]}"]
  2 -> 4 [label="c & !b\\n{Acc[1]}"]
  2 -> 6 [label="b & !c\\n{Acc[1]}"]
  3 [label="2"]
  3 -> 3 [label="a & c\\n{Acc[1]}"]
  3 -> 4 [label="c & !a\\n{Acc[1]}"]
  3 -> 7 [label="a & !c\\n{Acc[1]}"]
  4 [label="0"]
  4 -> 4 [label="c\\n{Acc[1]}"]
  5 [label="4"]
  5 -> 5 [label="a & b\\n{Acc[1]}"]
  5 -> 6 [label="b & !a\\n{Acc[1]}"]
  5 -> 7 [label="a & !b\\n{Acc[1]}"]
  6 [label="3"]
  6 -> 6 [label="b\\n{Acc[1]}"]
  7 [label="5"]
  7 -> 7 [label="a\\n{Acc[1]}"]
}
#+end_example

#+BEGIN_SRC dot :file gagbgc2.png :cmdline -Tpng :var txt=gagbgc2 :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:gagbgc2.png]]

You can augment the number of terms in the disjunction to magnify the
difference.  For N terms, the =--small= automaton has N+1 states,
while the =--deterministic= automaton needs 2^N-1 states.

A last parameter that can be used to tune the translation is the amount
of pre- and post-processing performed.  These two steps can be adjusted
via a common set of switches:
#+BEGIN_SRC sh :results verbatim :exports results
ltl2tgba --help | sed -n '/Optimization level:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
:       --high                 all available optimizations (slow, default)
:       --low                  minimal optimizations (fast)
:       --medium               moderate optimizations

Pre-processings are rewritings done on the LTL formulas, usually to
reduce its size, but mainly to put it in a form that will help the
translator (for instance =F(a|b)= is easier to translate than
=F(a)|F(b)=).  At =--low= level, only simple syntactic rewritings are
performed.  At =--medium= level, additional simplifications based on
syntactic implications are performed.  At =--high= level, language
containment is used instead of syntactic implications.

Post-processings are cleanups and simplifications of the automaton
produced by the core translator.  The algorithms used during post-processing
are
- SCC filtering: removing useless strongly connected components,
  and useless acceptance sets.
- direct simulation: merge states based on suffix inclusion.
- iterated simulations: merge states based on suffix inclusion,
  or prefix inclusion, in a loop.
- WDBA minimization: determinize and minimize automata representing
  obligation properties.
- degeneralization: convert a TGBA into a BA

The chaining of these various algorithms depends on the selected
combination of optimization level (=--low=, =--medium=, =--high=),
translation intent (=--small=, =--deterministic=) and type of
automaton desired (=--tgba=, =--ba=).

A notable configuration is =--any --low=, which will produce a TGBA as
fast as possible.  In this case, post-processing is disabled, and only
syntactic rewritings are performed.  This can be used for
satisfiability checking, although in this context even building an
automaton is overkill (you only need an accepted run).

Finally, it should be noted that the default optimization options
(=--small --high=) are usually overkill.  =--low= will produce good
automata most of the time.  Most of pattern formulas of [[file:genltl.org][=genltl=]] will
be efficiently translated in this configuration (meaning that =--small
--high= will not produce a better automaton).  If you are planning to
generate automata for large family of pattern formulas, it makes sense
to experiment with the different settings on a small version of the
pattern, and select the lowest setting that satisfies your
expectations.

* Translating multiple formulas for statistics

If multiple formulas are given to =ltl2tgba=, the corresponding
automata will be output one after the other.  This is not very
convenient, since most of these output formats are not designed to
represent multiple automata, and tools like =dot= will only display
the first one.

One situation where passing many formulas to =ltl2tgba= is useful is
in combination with the =--stats=FORMAT= option.  This option will
output statistics about the translated automata instead of the
automata themselves.  The =FORMAT= string should indicate which
statistics should be output, and how they should be output using the
following sequence of characters (other characters are output as-is):

#+BEGIN_SRC sh :results verbatim :exports results
ltl2tgba --help | sed -n '/^ *%/p'
#+END_SRC
#+RESULTS:
:   %%                         a single %
:   %a                         number of acceptance sets
:   %c                         number of SCCs
:   %d                         1 if the automaton is deterministic, 0 otherwise
:   %e                         number of edges
:   %f                         the formula, in Spot's syntax
:   %n                         number of nondeterministic states
:   %s                         number of states
:   %t                         number of transitions

For instance we can study the size of the automata generated for the
right-nested =U= formulas as follows:

#+BEGIN_SRC sh :results verbatim :exports both
genltl --u-right=1..8 | ltl2tgba -F - --stats '%s states and %e edges for "%f"'
#+END_SRC
#+RESULTS:
: 2 states and 2 edges for "p1"
: 2 states and 3 edges for "p1 U p2"
: 3 states and 6 edges for "p1 U (p2 U p3)"
: 4 states and 10 edges for "p1 U (p2 U (p3 U p4))"
: 5 states and 15 edges for "p1 U (p2 U (p3 U (p4 U p5)))"
: 6 states and 21 edges for "p1 U (p2 U (p3 U (p4 U (p5 U p6))))"
: 7 states and 28 edges for "p1 U (p2 U (p3 U (p4 U (p5 U (p6 U p7)))))"
: 8 states and 36 edges for "p1 U (p2 U (p3 U (p4 U (p5 U (p6 U (p7 U p8))))))"

Here =-F -= means that formulas should be read from the standard input.

When computing the size of an automaton, we distinguish /transitions/
and /edges/.  An edge between two states is labeled by a Boolean
formula and may in fact represent several transitions labeled by
compatible Boolean assignment.

For instance if the atomic propositions are =x= and =y=, an edge labeled
by the formula =!x= actually represents two transitions labeled respectively
with =!x&y= and =!x&!y=.

Two automata with the same structures (states and edges) but differing
labels, may have a different count of transitions, e.g., if one has
more restricted labels.

* Building Monitors

In addition to TGBA and BA, =ltl2tgba= can output /monitor/ using the
=-M= option.  These are finite automata that accept all prefixes of a
formula.  The idea is that you can use these automata to monitor a
system as it is running, and report a violation as soon as no
compatible outgoing transition exist.

=ltl2tgba -M= may output non-deterministic monitors while =ltl2tgba
-MD= (short for =--monitor --deterministic=) will output the minimal
deterministic monitor for the given formula.

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba -M '(Xa & Fb) | Gc'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1", peripheries=2]
  1 -> 2 [label="1\n"]
  1 -> 3 [label="c\n"]
  2 [label="2", peripheries=2]
  2 -> 4 [label="a\n"]
  3 [label="3", peripheries=2]
  3 -> 3 [label="c\n"]
  4 [label="4", peripheries=2]
  4 -> 4 [label="1\n"]
}
#+end_example
#+NAME: monitor1
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba -M '(Xa & Fb) | Gc' | sed 's/\\/\\\\/'
#+END_SRC

#+RESULTS: monitor1
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1", peripheries=2]
  1 -> 2 [label="1\\n"]
  1 -> 3 [label="c\\n"]
  2 [label="2", peripheries=2]
  2 -> 4 [label="a\\n"]
  3 [label="3", peripheries=2]
  3 -> 3 [label="c\\n"]
  4 [label="4", peripheries=2]
  4 -> 4 [label="1\\n"]
}
#+end_example

#+BEGIN_SRC dot :file monitor1.png :cmdline -Tpng :var txt=monitor1 :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:monitor1.png]]

#+BEGIN_SRC sh :results verbatim :exports code
ltl2tgba -MD '(Xa & Fb) | Gc'
#+END_SRC
#+RESULTS:
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1", peripheries=2]
  1 -> 2 [label="c\n"]
  1 -> 3 [label="!c\n"]
  2 [label="4", peripheries=2]
  2 -> 4 [label="a\n"]
  2 -> 5 [label="c & !a\n"]
  3 [label="3", peripheries=2]
  3 -> 4 [label="a\n"]
  4 [label="2", peripheries=2]
  4 -> 4 [label="1\n"]
  5 [label="0", peripheries=2]
  5 -> 5 [label="c\n"]
}
#+end_example

#+NAME: monitor2
#+BEGIN_SRC sh :results verbatim :exports none
ltl2tgba -MD '(Xa & Fb) | Gc' | sed 's/\\/\\\\/'
#+END_SRC

#+RESULTS: monitor2
#+begin_example
digraph G {
  0 [label="", style=invis, height=0]
  0 -> 1
  1 [label="1", peripheries=2]
  1 -> 2 [label="c\\n"]
  1 -> 3 [label="!c\\n"]
  2 [label="4", peripheries=2]
  2 -> 4 [label="a\\n"]
  2 -> 5 [label="c & !a\\n"]
  3 [label="3", peripheries=2]
  3 -> 4 [label="a\\n"]
  4 [label="2", peripheries=2]
  4 -> 4 [label="1\\n"]
  5 [label="0", peripheries=2]
  5 -> 5 [label="c\\n"]
}
#+end_example

#+BEGIN_SRC dot :file monitor2.png :cmdline -Tpng :var txt=monitor2 :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:monitor2.png]]

Because they accept all finite executions that could be extended to
match the formula, monitor cannot be used to check for eventualities
such as =F(a)=.  Any finite execution can be extended to match =F(a)=.

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