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Fix interpretation of {e[*]} and !{e[*]}.

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This follows from a discussion with Ernesto Posse.

The semantics for the {...} operator we use in Spot comes from the
cl(...) operator defined by Dax et al. (ATVA'09).  This is slightly
different from the the way the PSL spec interprets a SERE used in the
context of a temporal formula (appendix B.3.1.1.2, item 7).

cl({a;b}[*]) would match any infinite word that starts with a;b, while
in PSL {a;b}[*] would match any infinite word that alternates a and b.

Spot documents that {SERE} in a temporal formula is interpreted like
cl(SERE) however it failed to ignore the empty prefix of SERE.  So
{{a;b}[*]} would match anything, because the empty word is a prefix of
any word, and is also accepted by {a;b}[*].  Some trivial identities
and basic rewritings were also wrongly considering these empty
prefixes as well.

This patch therefore fixes the translation and syntactic
simplification rules, to really ignore these empty prefixes.

In some future version it should probably be wise to rename this {...}
operator as cl(...), and use {...} for the semantics given in appendix
B.3.1.1.2 (item 7) of the PSL specs.

* src/ltlast/unop.cc: Fix trivial identities.  We have
{[*0]} = 0 and !{[*0]} = 1.
* src/ltlvisit/simplify.cc: Fix basic rewriting rules.
{e[*]} = {e} and !{e[*]} = !{e}.
* doc/tl/tl.tex: Adjust documentation.
* doc/tl/tl.bib (dax.09.atva): New entry.
* src/tgbaalgos/ltl2tgba_fm.cc: Do not accept any
infinite word for {e[*]} just because the empty
prefix is matched by e[*].
* src/tgbatest/ltl2tgba.test: Add a test case.
* NEWS: Mention it.
* THANKS: Add Ernesto.
This commit is contained in:
Alexandre Duret-Lutz 2013-07-28 21:04:10 +02:00
parent 4f986400f1
commit cb7bdf8c1f
8 changed files with 103 additions and 44 deletions
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@ -7,6 +7,13 @@ New in spot 1.1.3a (not released)
- When used from ltlcross, the same parser would fail to
parse further neverclaims after the first failure.
- Add a missing newline in some error message of ltlcross.
- Expressions like {SERE} were wrongly translated and simplified
for SEREs that accept the empty word: they were wrongly reduced
to true. Simplification and translation rules have been fixed,
and the doc/tl/tl.pdf specifications have been updated to better
explain that {SERE} has the semantics of a closure operator that
is not exactly what one could expect after reading the PSL
standard.
New in spot 1.1.3 (2013-07-09)