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.
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