* mini_quip: port to Python 3 and fix several bugs
examples/python/mini_quip.py was Python 2 only and had several
latent bugs that prevented it from running on Python 3 or producing
correct results on benchmarks beyond horn1..5.
Python 3 / import fixes:
- Convert `print stmt` to `print(...)` calls (lines 457-458, 567,
710, 747, 765, 776).
- The bare `print("Test file: %s") % file` form was applying `%`
to the return value of print() (None); rewrite as
`print("Test file: %s" % file)`.
- Add `import sys` (used by sys.stdout.write/flush) and
`import copy` (used by QReach.state2cube via copy.deepcopy);
neither was previously imported.
- next()/prev() passed `zip(...)` directly to z3.substitute. In
Python 3 zip returns a one-shot generator; wrap with list() the
same way mini_ic3 already does.
Bug fixes:
- is_transition(): when an init rule's body is an And without any
Invariant predicate, is_body() returns (And(...), None). The
function then passed inv0=None to subst_vars and crashed inside
get_vars(). Add an explicit None check so the rule falls through
to is_init() (same fix as mini_ic3).
- generalize(): guard against an empty unsat core. Without the
guard, an empty core can be returned and become
cube2clause([])=Or([])=False, poisoning all frames (same class
of bug as in mini_ic3).
- check_reachable(): self.prev(cube) on an empty cube produced an
empty list which was then added to a solver as a no-op
constraint, so an empty cube would always look reachable. Only
add the constraint when cube is non-empty.
- quip_blocked() at f==0 for must goals contained
`assert is_sat == s.check()` where `is_sat` is undefined in that
scope; the intent is `assert sat == s.check()`.
- Inside the lemma-pushing loop in quip_blocked(), `is_sat == unsat`
was a comparison whose result was discarded; the intended
assignment is `is_sat = unsat`.
Verified on horn1..5 (unchanged behavior, all return same
SAFE/UNSAFE result and validate). Larger benchmarks (h_CRC,
h_FIFO, cache_coherence_three) now at least run without exceptions
(performance is a separate matter).
* mini_quip: guard against None from QReach.intersect in CEX trace loop
In quip_blocked, the must-goal CEX-tracing loop calls
self.reachable.intersect(self.prev(r)) and immediately uses
r.children() on the result. QReach.intersect can return None when
the model literals do not match any state in the partial reachable
set, which crashes with AttributeError: 'NoneType' object has no
attribute 'children'. Reproduces on data/h_FIFO.smt2.
Fix: save the model, and when intersect returns None fall back to
the raw self.project0(model) as the predecessor cube. This still
gives a concrete predecessor and lets the CEX trace make progress
instead of crashing.
