if e is pure eventuality and g => e, then e U g = Fg
if u is purely universal and u => g, then u R g = Gg
Fixes#93.
* doc/tl/tl.tex, NEWS: Document the rules.
* spot/tl/simplify.cc: Implement them.
* tests/core/reduccmp.test: Test them.
* tests/core/det.test: Adjust.