This was prompted by reports by Andrew Wells and Yong Li.
* NEWS, doc/tl/tl.tex: Document the changes.
* THANKS: Add Andrew.
* bin/ltlfilt.cc: Match --ltl before --from-ltlf if needed.
* spot/parsetl/parsedecl.hh, spot/parsetl/parsetl.yy,
spot/parsetl/scantl.ll: Parse X[!].
* spot/tl/formula.cc, spot/tl/formula.hh: Declare the new operator.
* spot/tl/ltlf.cc: Adjust to handle op::X and op::strong_X correctly.
* spot/tl/dot.cc, spot/tl/mark.cc, spot/tl/mutation.cc,
spot/tl/print.cc, spot/tl/simplify.cc, spot/tl/snf.cc,
spot/tl/unabbrev.cc, spot/twa/formula2bdd.cc,
spot/twaalgos/ltl2taa.cc, spot/twaalgos/ltl2tgba_fm.cc,
tests/core/ltlgrind.test, tests/core/rand.test,
tests/core/sugar.test, tests/python/randltl.ipynb: Adjust.
* tests/core/ltlfilt.test, tests/core/sugar.test,
tests/core/utf8.test: More tests.
The first issue was reported by Victor Khomenko.
* spot/tl/formula.cc: Introduce a single-argument
version of sugar_delay().
* spot/parsetl/parsetl.yy: Use it.
* doc/tl/tl.tex, spot/tl/formula.hh: Adjust doc.
* tests/core/ltlfilt.test, tests/core/sugar.test: More tests.
* spot/tl/formula.cc, spot/tl/formula.hh (formula::sugar_delay): New
function to implement this operator as syntactic sugar.
* spot/parsetl/parsetl.yy, spot/parsetl/scantl.ll: Parse it.
* doc/tl/tl.tex: Document the syntactic sugar rules and precedence.
* tests/core/sugar.test: Add tests.
* NEWS: Mention this new feature.
* NEWS, doc/tl/tl.tex, doc/tl/tl.bib: Document these new operators.
* spot/parsetl/parsetl.yy, spot/parsetl/scantl.ll: Parse those.
* spot/tl/formula.cc, spot/tl/formula.hh: Add constructors.
* spot/gen/formulas.cc: Use it.
* tests/core/sugar.test: New file.
* tests/Makefile.am: Add it.
