1. Manager decides worker’s lease is stale.
2. It records/sends lease cancel.
3. Worker finishes naturally before observing
4. Worker releases/discards old lease.
5. The lease-cancel counter is still sitting on the worker limit.
6. Later, a naked m.inc() check sees that stale cancel and treats it as global cancellation.
The C++ implementation of the fixedpoint engine (in
z3/src/api/api_datalog.cpp) already attempts to read `rlimit` from its
local parameters:
```c++
unsigned rlimit = to_fixedpoint(d)->m_params.get_uint("rlimit", mk_c(c)->get_rlimit());
```
However, because `rlimit` was not registered in the public fp parameter
schema (`fp_params.pyg`), any attempt by clients to set it locally via
`Z3_fixedpoint_set_params` was rejected by the Z3 parameter validator
with an "unknown parameter" error.
Implements the algorithm of Eq(p,q) = Empty(p XOR q)' using a union-find
driven bisimulation closure (per the CAV'26 ERE paper).
### What's added
* **New primitive OP_RE_XOR (re.xor)** wired through seq_decl_plugin:
parser signature, info propagation (nullable, min_length), and
pretty-printer.
* **seq_rewriter**: structural XOR rewrites ( XOR r = empty, XOR empty =
r, ull XOR r = comp(r), comp/comp absorption, complement push, AC
normalisation), nullability (Null(p XOR q) = Null(p) != Null(q)),
derivative (D_a(p XOR q) = D_a(p) XOR D_a(q)), reverse, antimirov
derivative, and `check_deriv_normal_form` coverage.
* **New class seq::regex_bisim** in
`src/ast/rewriter/seq_regex_bisim.{h,cpp}` to keep the bisim logic out
of the already-large `seq_rewriter.cpp`. Uses `basic_union_find` from
`util/union_find.h`, an `obj_map` for the node assignment, and a
50000-step bound (returns `l_undef` on overrun).
* **Integration** in `seq_rewriter::reduce_re_eq` (with a re-entry
guard) and in `seq_regex::propagate_eq` / `propagate_ne` for ground
regexes; on `l_undef` we fall back to the existing axiomatisation.
* **`sls_seq_plugin`**: extend `OP_RE_DIFF` switch arms to also cover
`OP_RE_XOR`.
### Validation
* Full release build with MSVC + Ninja.
* `./test-z3 /a` -- 89/89 tests passing.
* `./test-z3 /seq smt2print_parse` -- PASS.
* Smoke tests with `(a|b)*` vs `(a*b*)*` (equal) and `a*` vs `(a|b)*`
(not equal) return the expected `sat`/`unsat` quickly.
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
