not a formula&.  This is more homogeneous.
* src/ltlvisit/dump.cc (dump): Likewise.
* src/ltlvisit/dotty.hh (dotty): Likewise.
* src/ltlvisit/dotty.cc (dotty): Likewise.
* src/ltlvisit/tostring.hh (to_string): Likewise.
* src/ltlvisit/tostring.cc (to_string): Likewise.
* src/ltltest/readltl.cc, src/ltltest/equals.cc,
src/ltltest/tostring.cc: Adjust usage.

src/ltlast/constant.hh, src/ltlast/formula.hh,
src/ltlast/multop.hh, src/ltlast/unop.hh, src/ltlast/visitor.hh,
src/ltlenv/defaultenv.hh, src/ltlenv/environment.hh,
src/ltlparse/public.hh, src/ltlvisit/clone.hh,
src/ltlvisit/dotty.hh, src/ltlvisit/dump.hh,
src/ltlvisit/equals.hh, src/ltlvisit/lunabbrev.hh,
src/ltlvisit/nenoform.hh, src/ltlvisit/tunabbrev.hh: Add
Doxygen comments.
* src/visitor.hh: Do not use const_sel.  This clarify
the code and helps Doxygen.

* src/ltlast/formula.hh: ... this.
* src/ltlast/Makefile.am (libltlast_a_SOURCES): Adjust.
* src/ltlast/formula.hh (formulae): Rename as ...
(formula): ... this.
Adjust all uses.

src/Makefile.am, src/ltlast/Makefile.am, src/ltlast/allnodes.hh,
src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/binop.cc, src/ltlast/binop.hh, src/ltlast/constant.cc,
src/ltlast/constant.hh, src/ltlast/formulae.hh,
src/ltlast/multop.cc, src/ltlast/multop.hh, src/ltlast/predecl.hh,
src/ltlast/unop.cc, src/ltlast/unop.hh, src/ltlast/visitor.hh,
src/ltlparse/Makefile.am, src/ltlparse/ltlparse.yy,
src/ltlparse/ltlscan.ll, src/ltlparse/parsedecl.hh,
src/ltlparse/public.hh, src/ltlvisit/Makefile.am,
src/ltlvisit/dotty.cc, src/ltlvisit/dotty.hh,
src/ltlvisit/dump.cc, src/ltlvisit/dump.hh,
src/ltlvisit/rewrite.cc, src/ltlvisit/rewrite.hh,
src/ltltest/Makefile.am, src/ltltest/defs.in, src/ltltest/readltl.cc,
src/ltltest/parse.test, src/ltltest/parseerr.test,
src/misc/Makefile.am, src/misc/const_sel.hh: New files.