So far I have only checked these rewritings with reduccmp.test.
There are probably a few kinks to iron out.

* src/ltlvisit/simplify.cc: Reimplement most of the basic
rewriting rules, leaving some FIXME comments for dubious ones.
* src/ltlast/multop.cc, src/ltlast/multop.hh: Ignore NULL
pointers in the vector.
* src/ltlvisit/reduce.cc (reduce): Do not call basic_reduce().
* src/ltltest/reduccmp.test: Adjust tests.