two unabbreviate_logic definitions (const and non-const) into a
function that takes a const formula* and return a non-const
formula*. Since formula* is convertible to const formula*, and
the const version of the function just called the non-onst one, it
makes no sense to keep both. Also, it confused Swig.
* src/ltlvisit/nenoform.cc, src/ltlvisit/nenoform.hh: Likewise
for negative_normal_form.
* src/ltlvisit/tunabbrev.cc, src/ltlvisit/tunabbrev.hh: Likewise
for unabbreviate_ltl.
* src/ltlvisit/clone.cc, src/ltlvisit/clone.hh: Likewise for clone.
* src/ltlvisit/destroy.cc, src/ltlvisit/destroy.hh: Likewise
for destroy.
reference-counted formula*. One can't call constructors for AST
items anymore, everything need to be acquired through instance()
class methods.
* src/ltlast/formula.cc, src/ltlast/refformula.cc,
src/ltlast/refformula.hh: New files.
* src/ltlast/Makefile.am (libltlast_la_SOURCES): Add them.
* src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/unop.cc, src/ltlast/unop.hh,
src/ltlast/binop.cc, src/ltlast/binop.hh: Make the constructor
and destructor protected. Define a static function `instance()'
to get an instance with specific argument. Use a map called
`instances' to store all known instances. Inherit from
ref_formula.
* src/ltlast/constant.hh, src/ltlast/constant.cc: Protect
the constructor and destructor. Provide the false_instance()
and true_instance() functions instead.
* src/formula.hh (ref, unref, ref_, unref_): New methods.
* src/ltlast/multop.cc, src/ltlast/multop.hh: Protect
the constructor, destructor, as well as the add() method.
Provides the instance(), and add() class methods instead.
Store children_ as a pointer.
* src/ltlenv/defaultenv.cc (require): Adjust to
call atomic_prop::instance.
* src/ltlparse/ltlparse.yy: Adjust to call instance() functions
instead of constructors.
* src/ltltest/Makefile.am (LDADD): Tweak library ordering.
* src/ltlvisit/clone.hh (clone_visitor): Inherit from visitor,
not const_visitor, and adjust all prototypes appropriately.
* src/ltlvisit/clone.cc (clone_visitor): Likewise.
Call ref() or instance() methods instead of copy constructors.
* src/ltlvisit/equals.cc: Simplify atomic_prop and constant
cases.
* src/ltlvisit/lunabbrev.hh, src/ltlvisit/lunabbrev.cc,
src/ltlvisit/tunabbrev.hh, src/ltlvisit/tunabbrev.cc,
src/ltlvisit/nenoform.hh, src/ltlvisit/nenoform.cc: Use instance()
methods instead of constructor. Make these children of visitor, not
const_visitor.
* src/ltltest/readltl.c (main): Do not delete the formula.
