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.