# miniKanren-on-SX: relational programming on the CEK/VM

miniKanren is not a language to parse — it is an **embedded DSL** implemented as a library
of SX functions. No tokenizer, no transpiler. The entire system is a set of `define` forms
in `lib/minikanren/`. Programs are SX expressions using the miniKanren API.

The unique angle: SX's delimited continuation machinery (`perform`/`cek-resume`, call/cc)
maps almost perfectly to the search monad miniKanren needs. Backtracking is cooperative
suspension, not a separate trail machine. This is the cleanest possible host for miniKanren.

End-state goal: **full core miniKanren** (`run`, `fresh`, `==`, `conde`, `condu`, `onceo`,
`project`, `matche`) + **core.logic-style relations** (`appendo`, `membero`, `listo`,
`numbero`, etc.) + **arithmetic constraints** (`fd` domain, `CLP(FD)` subset).

## Ground rules

- **Scope:** only touch `lib/minikanren/**` and `plans/minikanren-on-sx.md`. Do **not**
  edit `spec/`, `hosts/`, `shared/`, or other `lib/<lang>/`.
- **Shared-file issues** go under "Blockers" below with a minimal repro; do not fix here.
- **SX files:** use `sx-tree` MCP tools only.
- **Architecture:** pure library — no source parser. Programs are written in SX using the API.
- **Reference:** *The Reasoned Schemer* (Friedman/Byrd/Kiselyov) + Byrd's dissertation.
- **Commits:** one feature per commit. Keep `## Progress log` updated and tick boxes.

## Architecture sketch

```
SX program using miniKanren API
    │
    ├── lib/minikanren/unify.sx      — terms, variables, walk, unification, occurs check
    ├── lib/minikanren/substitution.sx — substitution as association list / hash table
    ├── lib/minikanren/stream.sx     — lazy streams of substitutions (via delay/force)
    ├── lib/minikanren/goals.sx      — == / fresh / conde / condu / onceo / project / matche
    ├── lib/minikanren/run.sx        — run* / run n — drive the search, extract answers
    ├── lib/minikanren/relations.sx  — standard relations: appendo, membero, listo, etc.
    └── lib/minikanren/clpfd.sx      — arithmetic constraints (CLP(FD) subset)
```

Key semantic mappings:
- **Logic variable** → SX vector of length 1 (mutable box); `make-var` creates fresh one;
  `walk` follows the substitution chain
- **Substitution** → SX association list (or hash table for performance) mapping var → term
- **Stream of substitutions** → lazy list using `delay`/`force` (Phase 9 of primitives)
- **Goal** → SX function `substitution → stream-of-substitutions`
- **`==`** → unifies two terms, extending substitution or failing (empty stream)
- **`fresh`** → introduces new logic variables; `(fresh (x y) goal)` → goal with x, y bound
- **`conde`** → interleave streams from multiple goal clauses (depth-first with interleaving)
- **`run n`** → drive the stream, collect first n substitutions, reify answers

## Roadmap

### Phase 1 — variables + unification
- [x] `make-var` → fresh logic variable (unique mutable box)
- [x] `var?` `v` → bool — is this a logic variable?
- [x] `walk` `term` `subst` → follow substitution chain to ground term or unbound var
- [x] `walk*` `term` `subst` → deep walk (recurse into lists/dicts)
- [x] `unify` `u` `v` `subst` → extended substitution or `#f` (failure)
      Handles: var/var, var/term, term/var, list unification, number/string/symbol equality.
      No occurs check by default; `unify-check` with occurs check as opt-in.
- [x] Empty substitution `empty-s` (dict-based via kit's `empty-subst` — assoc list was a sketch; kit ships dict, kept it)
- [x] Tests in `lib/minikanren/tests/unify.sx`: ground terms, vars, lists, failure, occurs

### Phase 2 — streams + goals
- [x] Stream type: `mzero` (empty), `unit s` (singleton), `mk-mplus` (interleave),
      `mk-bind` (apply goal to stream). Names mk-prefixed because SX has a host
      `bind` primitive that silently shadows user defines.
- [x] Lazy streams via thunks: a paused stream is a zero-arg fn; mk-mplus suspends
      and swaps when its left operand is paused, giving fair interleaving.
- [x] `==` goal: `(fn (s) (let ((s2 (mk-unify u v s))) (if s2 (unit s2) mzero)))`
- [x] `==-check` — opt-in occurs-checked equality goal
- [x] `succeed` / `fail` — trivial goals
- [x] `conj2` / `mk-conj` (variadic) — sequential conjunction
- [x] `disj2` / `mk-disj` (variadic) — interleaved disjunction (raw — `conde`
      adds the implicit-conj-per-clause sugar later)
- [x] `fresh` — introduces logic variables inside a goal body. Implemented as a
      defmacro: `(fresh (x y) g1 g2 ...)` ⇒ `(let ((x (make-var)) (y (make-var)))
      (mk-conj g1 g2 ...))`. Also `call-fresh` for programmatic goal building.
- [x] `conde` — sugar over disj+conj, one row per clause; defmacro that
      wraps each clause body in `mk-conj` and folds via `mk-disj`. Notes:
      with eager streams ordering is left-clause-first DFS; true interleaving
      requires paused thunks (Phase 4 recursive relations).
- [x] `condu` — committed choice. defmacro folding clauses into a runtime
      `condu-try` walker; first clause whose head goal yields a non-empty
      stream commits its first answer, rest-goals run on that single subst.
- [x] `onceo` — `(stream-take 1 (g s))`; trims a goal's stream to ≤1 answer.
- [x] Tests: basic goal composition, backtracking, interleaving (110 cumulative)

### Phase 3 — run + reification
- [x] `run*` `goal` → list of all answers (reified). defmacro: bind q-name as
      fresh var, conj goals, take all from stream, reify each.
- [x] `run n` `goal` → list of first n answers (defmacro; n = -1 means all)
- [x] `reify` `term` `subst` → walk* + build reification subst + walk* again
- [x] `reify-s` → maps each unbound var (in left-to-right walk order) to a
      `_.N` symbol via `(make-symbol (str "_." n))`
- [x] `fresh` with multiple variables — already shipped Phase 2B.
- [x] Query variable conventions: `q` as canonical query variable (matches TRS)
- [x] Tests: classic miniKanren programs — `(run* q (== q 1))` → `(1)`,
      `(run* q (conde ((== q 1)) ((== q 2))))` → `(1 2)`,
      `(run* q (fresh (x y) (== q (list x y))))` → `((_.0 _.1))`. Peano +
      `appendo` deferred to Phase 4.

### Phase 4 — standard relations
- [x] `appendo` `l` `s` `ls` — list append, runs forwards AND backwards.
      Canary green: `(run* q (appendo (1 2) (3 4) q))` → `((1 2 3 4))`;
      `(run* q (fresh (l s) (appendo l s (1 2 3)) (== q (list l s))))` →
      all four splits.
- [x] `membero` `x` `l` — enumerates: `(run* q (membero q (a b c)))` → `(a b c)`
- [x] `listo` `l` — l is a proper list; enumerates list shapes with laziness
- [x] `nullo` `l` — l is empty
- [x] `pairo` `p` — p is a (non-empty) cons-cell / list
- [x] `caro` / `cdro` / `conso` / `firsto` / `resto`
- [x] `reverseo` `l` `r` — reverse of list. Forward is fast; backward is `run 1`-clean,
      `run*` diverges due to interleaved unbounded list search (canonical TRS issue).
- [x] `flatteno` `l` `f` — flatten nested lists. Three conde clauses:
      empty → empty; pair → recurse on car & cdr + appendo; otherwise atom →
      `(== flat (list tree))`. Atom-ness is asserted via `nafc (nullo tree)
      + nafc (pairo tree)` — both succeed only when tree is a ground non-list.
      Works on ground inputs.
- [x] `permuteo` `l` `p` — permutation of list. Built on `inserto` (insert at any
      position) and recursive permutation of tail. All 6 perms of (1 2 3) generated
      forward; backward `run 1 q (permuteo q (a b c))` finds the input.
- [x] `lengtho` `l` `n` — length as a relation, Peano-encoded:
      `:z` / `(:s :z)` / `(:s (:s :z))` ... matches TRS. Forward is direct;
      backward enumerates lists of a given length.
- [x] Tests: run each relation forwards and backwards (so far 25 in
      `tests/relations.sx`; reverseo/flatteno/permuteo/lengtho deferred)

### Phase 5 — `project` + `matche` + negation
- [x] `project` `(x ...) body` — defmacro: rebinds named vars to `(mk-walk* var s)`
      in the body's lexical scope, then runs `(mk-conj body...)` on the same
      substitution. Hygienic via gensym'd `s`-param. (`Phase 5 piece B`)
- [x] `matche` — pattern matching over logic terms. Pattern grammar: `_` /
      symbol / atom / `()` / `(p1 p2 ... pn)`. Each clause becomes
      `(fresh (vars-in-pat) (== target pat-expr) body...)`. Repeated symbol
      names in a pattern produce the same fresh var, so they unify (== check).
      Built without quasiquote (the expander does not recurse into lambda
      bodies). Fixed-length list patterns only — head/tail destructuring uses
      `(fresh (a d) (conso a d target) body)` directly.
- [x] `conda` — soft-cut: first non-failing head wins; ALL of head's answers
      flow through rest-goals; later clauses not tried (`Phase 5 piece A`)
- [x] `condu` — committed choice (Phase 2)
- [x] `nafc` — negation as finite failure: `(nafc g)` yields the input subst
      iff g has zero answers. Standard caveats apply (open-world unsoundness;
      diverges if g is infinite). `Phase 5 piece C`.
- [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`

### Phase 6 — arithmetic constraints CLP(FD)
- [x] Finite domain variables: domain stored under reserved key `_fd` in
      the substitution dict; ascending sorted-int-list representation;
      domain primitives `fd-dom-from-list`, `fd-dom-intersect`,
      `fd-dom-without`, `fd-dom-range`, `fd-dom-min/max/empty?/singleton?`.
- [x] `fd-in x dom` — narrows x's domain by intersection.
- [x] `fd-eq x y`, `fd-neq x y`, `fd-lt`, `fd-lte` — propagator-store
      goals. Each adds a closure to the constraints field and runs it
      on post; closures re-fire after every label step via fd-fire-store.
- [x] `fd-plus x y z`, `fd-times x y z` — ground-cases propagators
      (when 2 of 3 walk to numbers, the third is derived).
- [x] `fd-distinct vars` — pairwise alldifferent via fd-neq folds.
- [x] Constraint reactivation: `fd-fire-store` iterates to fixed point
      using a domain+bindings signature comparison; ensures multi-step
      propagation chains (e.g. fd-plus binds a fresh var, which then
      lets a downstream fd-neq fire).
- [x] Labelling: `fd-label vars` enumerates each var's domain via
      mk-mplus over singleton bindings; constraint store re-fires after
      each binding.
- [x] Tests: N-queens via FD — 4-queens finds both solutions, 5-queens
      finds all 10 in seconds (vs the naive enumerate-then-filter
      version which times out past N=4).
- [x] `ino` `x` `domain` — alias for `(membero x domain)` (kept for
      the simple enumerate-then-filter pattern alongside fd-in).
- [x] `all-distincto` `l` — original membero-based version (kept alongside
      the newer fd-distinct).
- [ ] `fd-eq` `x` `y` — x = y (constraint propagation)
- [ ] `fd-neq` `x` `y` — x ≠ y
- [ ] `fd-lt` `fd-lte` `fd-gt` `fd-gte` — ordering constraints
- [ ] `fd-plus` `x` `y` `z` — x + y = z (constraint)
- [ ] `fd-times` `x` `y` `z` — x * y = z
- [ ] Arc consistency propagation — when domain narrows, propagate to constrained vars
- [ ] Labelling: `fd-run` drives search by splitting domains when propagation stalls
- [ ] Tests: send-more-money, N-queens with CLP(FD), map coloring, cryptarithmetic

### Phase 7 — tabling (memoization of relations)
- [x] `table-1`, `table-2`, `table-3` wrappers: ground-arg memoization
      for 1-, 2-, and 3-argument relations.
- [x] `table-2` wrapper: ground-arg memoization for 2-arg relations.
      Cache keyed by walked input; on miss runs underlying relation,
      collects all output values from the answer stream, stores, and
      replays. Subsequent calls with the same ground input replay the
      cached values (no recomputation).
- [x] Fibonacci canary green: tabled `fib(25) = 75025` in seconds;
      naive `fib(25)` times out at 60s. Memoization turns exponential
      recursion into linear.
- [x] Ackermann canary green via `table-3`: `A(3, 3) = 61`.
- [ ] Producer/consumer SLG scheduling — required to handle recursive
      tabled calls with the SAME ground key (e.g. cyclic `patho` with a
      shared key); naive memoization deferred to a future iteration.
- [ ] Tests: cyclic graph reachability via tabled patho (deferred —
      requires SLG); mutual recursion (deferred).

## Blockers

_(none yet)_

## Progress log

_Newest first._

- **2026-05-08** — **Session snapshot**: 17 lib files, 61 test files, 1229
  library LOC + 4360 test LOC, **551/551 tests cumulative**. Library covers
  Phases 1–5 fully, Phase 6 partial (FD helpers + intarith escape), Phase 7
  documented via cyclic-graph divergence test. lib-guest validation
  completed: `lib/minikanren/unify.sx` ≈ 50 LOC of local logic over
  `lib/guest/match.sx`'s ≈100 LOC kit (kit earns ~3× by line count). Major
  classic miniKanren tests green: appendo forwards/backwards, Peano
  arithmetic, 4-queens, Pythagorean triples, family-relations / pet
  puzzle / symbolic differentiation, 2x2 Latin square. Ready for Phase 6
  (native FD with arc-consistency) and Phase 7 (tabling) as future work.
- **2026-05-08** — **zip-with-o**: element-wise relational combine over two
  lists with a 3-arg combiner. Ground-only by composition. 5 new tests.
- **2026-05-08** — **take-while-o + drop-while-o**: predicate-driven
  prefix/suffix split. Roundtrip property verified. 8 new tests.
- **2026-05-08** — **arith-progo**: arithmetic-progression list generator
  via project. 6 new tests.
- **2026-05-08** — **counto**: count occurrences of x in l (intarith).
  6 new tests.
- **2026-05-08** — **nub-o**: dedupe via membero-on-tail. Multiplicity
  caveat documented in tests. 5 new tests.
- **2026-05-08** — **simplify-step-o**: algebraic identity simplifier
  (conda demo). 6 new tests.
- **2026-05-08** — **flat-mapo**: concatMap-style relation. 5 new tests.
- **2026-05-08** — **foldl-o (relational left fold)**: complement to foldr-o. Combiner has args (acc, head) -> new-acc. (foldl-o pluso-i (1 2 3 4 5) 0 q) -> 15; (foldl-o flipped-conso l () q) reverses l. 5 new tests, 510/510 cumulative.
- **2026-05-08** — **foldr-o (relational right fold)**: takes a 3-arg combiner relation rel, a list, an initial accumulator, produces the result. (foldr-o appendo lists () q) is a flatten; (foldr-o conso l () q) rebuilds l. 4 new tests, 505/505 cumulative.
- **2026-05-08** — **enumerate-i / enumerate-from-i — 500-test milestone**: index-each-element relations. (enumerate-i l result) -> result is l with each element paired with its 0-based index. (enumerate-from-i n l result) starts at n. 5 new tests, **501/501** cumulative.
- **2026-05-08** — **partitiono**: relational partition. (partitiono pred l yes no) splits l so yes contains elements where pred succeeds and no contains the rest. Composes with intarith for numeric predicates. 5 new tests, 496/496 cumulative.
- **2026-05-08** — **appendo3**: 3-list append via two appendos. (appendo3 a b c r) is (appendo a b mid) ∧ (appendo mid c r). Recovers any of the three lists given the other two and the result. 5 new tests, 491/491 cumulative.
- **2026-05-08** — **lengtho-i**: integer-indexed length (intarith). Empty list -> 0; recurse with pluso-i. Drop-in fast replacement for Peano lengtho when the count fits in a host integer. 5 new tests, 486/486 cumulative.
- **2026-05-08** — **sumo + producto (intarith)**: fold a list of ground integers to its sum or product. Empty list -> identity (0 / 1). Recurse via pluso-i / *o-i. 9 new tests, 481/481 cumulative.
- **2026-05-08** — **mino + maxo (intarith)**: find the minimum/maximum of a non-empty list of ground integers. Two conde clauses each: singleton (return the element) / multi (compare head against recursive min/max of tail). 9 new tests, 472/472 cumulative.
- **2026-05-08** — **sortedo (intarith)**: list is non-decreasing under integer ordering. Three conde clauses: empty / singleton / pair-and-recurse. Uses lteo-i for the adjacent-pair check (ground integers). 7 new tests, 463/463 cumulative.
- **2026-05-08** — **subseto**: every element of l1 is a member of l2. Recurses on l1, checking each element via membero. Allows duplicates in l1 (each independently in l2). 7 new tests, 456/456 cumulative.
- **2026-05-08** — **cycle-free path search**: mitigation for the cyclic-graph divergence — thread a  accumulator through the recursion, gate each step with nafc + membero on visited. Terminates on graphs with cycles; no Phase-7 tabling required for the simple case. 3 new tests, 449/449 cumulative.
- **2026-05-08** — **removeo-allo**: removes every occurrence of x (vs rembero, which removes only the first). Three conde clauses: empty -> empty; head matches -> skip and recurse; head differs (nafc) -> keep and recurse. 5 new tests, 446/446 cumulative.
- **2026-05-08** — **btree-walko (matche showcase)**: walks a binary tree (:leaf v) | (:node l r) and emits each leaf value via conde. Demonstrates matche dispatch on tagged-list patterns, recursion through both branches via conde, and run* enumerating all 5 leaves of a small tree. 4 new tests, 441/441 cumulative.
- **2026-05-08** — **swap-firsto**: swap the first two elements of a list. Built via four conso constraints. Self-inverse on length-2+ lists; runs forward and backward. 6 new tests, 437/437 cumulative.
- **2026-05-08** — **pairlisto**: relational zip — pairs is the zipped list of (l1[i] l2[i]). Runs forward, recovers l1 given l2 and pairs, recovers l2 given l1 and pairs. 5 new tests, 431/431 cumulative.
- **2026-05-08** — **iterate-no**: relational iterated application. Applies a 2-arg relation n times (Peano n) starting from x to produce result. Generalises succ-iteration / list-cons-iteration / etc. 4 new tests, 426/426 cumulative.
- **2026-05-08** — **rev-acco / rev-2o**: accumulator-style reversal — faster than appendo-driven reverseo for forward queries because no quadratic appendos. Trade-off: rev-acco is asymmetric (cannot run cleanly backward in run*). 6 new tests, 422/422 cumulative.
- **2026-05-08** — **even-i / odd-i (intarith)**: ground-only parity goals via project. Composes with membero for filtered enumeration:  -> . 5 new tests, 416/416 cumulative.
- **2026-05-08** — **selecto**: classic miniKanren "choose an element + rest". Direct base (l = (x . rest)) plus skip-head recurse. Enumerates all (element, rest) splits in run*; runs forward, backward, mid-pipeline. 6 new tests, 411/411 cumulative.
- **2026-05-08** — **subo (contiguous sublist)**: Two appendos chained — l = front ++ s ++ back. Goal order matters: appendo on the ground l first, so the search is finitary; then constrain front. 7 new tests, 405/405 cumulative.
- **2026-05-08** — **prefixo + suffixo**: classic appendo-derived sublist relations. (prefixo p l) ≡ p ⊕ ? = l; (suffixo s l) ≡ ? ⊕ s = l. Both enumerate all prefixes/suffixes when given a fresh first arg. 9 new tests, 398/398 cumulative.
- **2026-05-08** — **palindromeo**: 2-line definition (reverseo + ==). Succeeds when a list reads the same forwards and backwards. 7 new tests, 389/389 cumulative.
- **2026-05-08** — **not-membero**: relational "x is not a member of l".
  Uses `nafc + ==` per element (the same skeleton all-distincto uses).
  Useful as a constraint-style filter: `(membero x dom) (not-membero x
  excluded)`. 4 new tests, 382/382 cumulative.
- **2026-05-08** — **repeato + concato**: repeato builds a list of n copies
  (Peano n); concato is fold-appendo over a list of lists. Both run forward
  and backward — repeato can recover the count from a uniform list. 10 new
  tests, 378/378 cumulative.
- **2026-05-08** — **tako + dropo (Peano-indexed prefix/suffix)**: takes / drops
  the first n elements via a Peano-encoded count. Round-trip
  `(tako n l) ⊕ (dropo n l) = l` holds. 9 new tests, 368/368 cumulative.
- **2026-05-08** — **eveno / oddo Peano predicates**: classic miniKanren parity
  relations. eveno: 0 or successor-of-successor of even; oddo: 1 or
  successor-of-successor of odd. Both run forward (test) and backward
  (enumerate). 12 new tests, 359/359 cumulative.
- **2026-05-08** — **defrel — Prolog-style relation definition macro**:
  `(defrel (NAME ARGS...) (CLAUSE1 ...) (CLAUSE2 ...) ...)` expands to
  `(define NAME (fn (ARGS...) (conde (CLAUSE1 ...) (CLAUSE2 ...) ...)))`.
  Mirrors Prolog's clause syntax and inherits Zzz-via-conde so recursive
  relations terminate. 3 new tests, 347/347 cumulative.
- **2026-05-08** — **lasto / init-o**: classic head/tail-style list relations.
  lasto extracts the final element; init-o extracts everything-but-the-last.
  Together with appendo, the round-trip `init ⊕ (last) = l` holds. 8 new
  tests, 344/344 cumulative.
- **2026-05-08** — **Datalog-style relational database queries**: tests/rdb.sx
  shows miniKanren as a query engine over fact tables. Defines two tables
  (employees + project assignments), wraps each with a relation that does
  membero over the table, then expresses queries: dept filter, salary >
  threshold (intarith), join engineers ↔ runtime project, find anyone on
  multiple distinct projects (nafc + ==). 5 new tests, 336/336 cumulative.
- **2026-05-08** — **Cyclic graph behaviour test (motivates Phase 7 tabling)**:
  Demonstrates that naive patho on a 2-cycle generates ever-longer paths.
  `run 5` truncates to a finite prefix; `run*` would diverge. This is
  exactly the test case Phase 7 (tabled / SLG resolution) is designed
  to fix. 3 new tests, 331/331 cumulative.
- **2026-05-08** — **numbero / stringo / symbolo (type predicates)**: ground-only
  type tests via project. Compose with `membero` for type-filtered enumeration:
  `(fresh (x) (membero x (1 "a" 2 "b" 3)) (numbero x) (== q x))` → `(1 2 3)`.
  Note: SX keywords are strings, so `(stringo :k)` succeeds. 12 new tests,
  328/328 cumulative.
- **2026-05-08** — **Graph reachability via patho**: classic miniKanren
  graph search. `edgeo` looks up edges in a fact list via `membero`; `patho`
  recursively builds paths via direct-edge OR (one edge + recurse + cons).
  Enumerates all paths between two nodes, including alternates through
  shortcuts. 6 new tests, 316/316 cumulative.
- **2026-05-08** — **everyo / someo (predicate-style relations)**:
  `(everyo rel l)` — every element of l satisfies rel; `(someo rel l)` —
  some element does. Both compose with intarith for numeric tests:
  `(everyo (fn (x) (lto-i x 10)) (list 1 5 9))` succeeds. 10 new tests,
  310/310 cumulative.
- **2026-05-08** — **mapo (relational map) — 300 test milestone**: `(mapo
  rel l1 l2)` succeeds when l2 is l1 with each element rel-related. Works
  forward and backward; composes with intarith — `(mapo (fn (a b) (*o-i
  a a b)) (1 2 3 4) q)` → `((1 4 9 16))`. 7 new tests, **300/300** cumulative.
- **2026-05-08** — **samelengtho**: classic miniKanren relation that
  succeeds when two lists have equal length. Symmetric — works to enumerate
  both lists fresh: `(run 3 q (fresh (l1 l2) (samelengtho l1 l2) (== q
  (list l1 l2))))` produces empty/empty, then 1-elem pairs, then 2-elem.
  5 new tests, 293/293 cumulative.
- **2026-05-08** — **Pythagorean triples (intarith showcase)**: search for
  (a, b, c) ∈ [1..10]³, a ≤ b, a² + b² = c² via `ino + lteo-i + *o-i +
  pluso-i + ==`. Finds exactly `(3 4 5)` and `(6 8 10)`. Demonstrates the
  enumerate-then-filter pattern with intarith escape into host arithmetic.
  1 new test, 288/288 cumulative.
- **2026-05-08** — **intarith.sx — fast ground-only integer arithmetic**:
  pluso-i / minuso-i / *o-i / lto-i / lteo-i / neqo-i wrap host arithmetic
  via `project`. They are not relational like Peano `pluso` (require args
  to walk to numbers), but run at native host speed — a viable escape
  hatch when puzzle size makes Peano impractical. Composes with relational
  goals: `(membero x dom) (lto-i x 3)` filters dom by `< 3`. 18 new tests,
  287/287 cumulative.
- **2026-05-08** — **2x2 Latin square**: small classic constraint demo using
  `ino` + 4 `all-distincto` constraints. Enumerates exactly 2 squares
  (`((1 2)(2 1))` and `((2 1)(1 2))`); a clue (top-left = 1) narrows to one.
  3 new tests, 269/269 cumulative. Note: 3x3 (12 squares) is the natural
  showcase but too slow under naive enumerate-then-filter — needs Phase 6
  arc-consistency.
- **2026-05-08** — **rembero / assoco / nth-o**: more standard list relations.
  rembero removes the first occurrence (uses `nafc (== a x)` to gate the
  skip clause, so it's well-defined on ground lists). assoco is alist
  lookup — works forward (key → value) and backward (value → key). nth-o
  uses Peano indices into a list. 13 new tests, 266/266 cumulative.
- **2026-05-08** — **flatteno**: nested-list flattener via conde + appendo.
  Atom-ness is detected via `nafc (nullo ...) + nafc (pairo ...)` so the
  third clause only fires when the input is a ground non-list. Works for
  `()` / atoms / flat lists / arbitrarily-nested lists. 7 new tests,
  253/253 cumulative. Phase 4 list relations all complete.
- **2026-05-08** — **Laziness tests**: explicitly verifies the
  Zzz-on-conde-clauses + mk-mplus-swap-on-paused machinery: an
  infinitely-recursive relation truncated via `run 4 q (listo-aux q)`
  produces exactly `(() (_.0) (_.0 _.1) (_.0 _.1 _.2))`; mixing two
  infinite generators via `mk-disj` keeps both alive (no starvation);
  `run*` terminates on a bounded query. 3 new tests, 246/246 cumulative.
- **2026-05-08** — **N-queens (classic miniKanren benchmark green)**:
  `lib/minikanren/queens.sx`. Encoding cols(i) = column of queen in row i;
  `ino-each` + `all-distincto` cover row/column constraints; `safe-diag`
  uses `project` to escape into host arithmetic for the `|c_i - c_j| ≠
  |i - j|` diagonal check; `all-cells-safe` walks pairs at construction
  time. `(run* q (fresh (a b c d) (== q (list a b c d)) (queens-cols
  (list a b c d) 4)))` returns the two valid 4-queens placements
  `(2 4 1 3)` and `(3 1 4 2)`. 6 new tests, 243/243 cumulative.
- **2026-05-08** — **Phase 6 piece A — minimal FD (ino + all-distincto)**:
  `lib/minikanren/fd.sx`. `ino` is `membero` with the FD-style argument
  order; `all-distincto` is `nafc + membero` walking the list. Together
  they cover the enumerate-then-filter style of finite-domain solving —
  `(fresh (a b c) (ino a D) (ino b D) (ino c D) (all-distincto (list a b c)))`
  enumerates all distinct triples from D. Full FD with arc-consistency,
  fd-plus etc. is still pending. 9 new tests, 237/237 cumulative.
- **2026-05-08** — **Classic puzzles + matche keyword fix**: matche now emits
  keywords bare in the pattern->expr conversion so they self-evaluate to their
  string name and unify with the same-keyword target value (instead of becoming
  a quoted-keyword type). New `tests/classics.sx`: pet permutation puzzle,
  parent/grandparent inference over a fact list, symbolic differentiation
  driven by matche dispatch on `:x` / `(:+ a b)` / `(:* a b)` patterns.
  6 new tests, 228/228 cumulative.
- **2026-05-08** — **Phase 5 piece D — matche, Phase 5 done**: pattern matching
  macro (`lib/minikanren/matche.sx`) — symbols become fresh vars, atoms become
  literals, lists recurse positionally, repeated names unify. 14 new tests
  (literals, vars, wildcards, list patterns, multi-clause dispatch, nested
  patterns, repeated-var-implies-eq). Built via `cons`/`list` rather than
  quasiquote because SX's quasiquote does not recurse into lambda bodies — a
  worth-knowing gotcha. 222/222 cumulative.
- **2026-05-08** — **Phase 4 piece C — permuteo + inserto**: standard recursive
  insert-at-any-position + permute-tail. 7 new tests, including all-6-perms-of-3
  as a set check. 208/208 cumulative.
- **2026-05-07** — **Phase 5 piece C — nafc**: `lib/minikanren/nafc.sx`. Three-line
  primitive: stream-take 1; if empty, `(unit s)`, else `mzero`. 7 tests including
  double-negation and use as a guard. 201/201 cumulative.
- **2026-05-07** — **Phase 5 piece B — project**: `lib/minikanren/project.sx` —
  defmacro that walks each named var, rebinds them, and runs the body's mk-conj.
  Demonstrated escape into host arithmetic / string ops (`(* n n)`, `(str s "!")`).
  Hygienic gensym'd s-param. 6 new tests, 194/194 cumulative.
- **2026-05-07** — **Peano arithmetic** (`lib/minikanren/peano.sx`): zeroo, pluso,
  minuso, lteo, lto, *o on Peano-encoded naturals (`:z` / `(:s n)`). pluso runs
  forward, backward, and enumerates: `(run* q (fresh (a b) (pluso a b 3)
  (== q (list a b))))` → all 4 pairs summing to 3. *o uses repeated pluso —
  works for small inputs, slower for larger. 19 new tests, 188/188 cumulative.
- **2026-05-07** — **Phase 5 piece A — conda**: soft-cut. Mirrors `condu` minus
  the `onceo` on the head: all head answers are conjuncted through the rest of
  the chosen clause. 7 new tests including the conda-vs-condu divergence test.
  169/169 cumulative.
- **2026-05-07** — **Phase 4 piece B — reverseo + lengtho**: reverseo runs forward
  cleanly and `run 1`-cleanly backward; lengtho uses Peano-encoded lengths so it
  works as a true relation in both directions (tests use the encoding directly).
  10 new tests, 162/162 cumulative.
- **2026-05-07** — **Phase 4 piece A — appendo canary green**: cons-cell support
  in `unify.sx` + `(:s head tail)` lazy stream refactor in `stream.sx` + hygienic
  `Zzz` (gensym'd subst-name) wrapping each `conde` clause + `lib/minikanren/
  relations.sx` with `nullo` / `pairo` / `caro` / `cdro` / `conso` / `firsto` /
  `resto` / `listo` / `appendo` / `membero`. 25 new tests in `tests/relations.sx`,
  152/152 cumulative.
  - **Three deep fixes shipped together**, all required to make `appendo`
    terminate in both directions:
    1. SX has no improper pairs, so a stream cell of mature subst + thunk
       tail can't use `cons` — moved to a `(:s head tail)` tagged shape.
    2. `(Zzz g)` wrapped its inner fn in a parameter named `s`, capturing
       the user goal's own `s` binding (the `(appendo l s ls)` convention).
       Replaced with `(gensym "zzz-s-")` for hygiene.
    3. SX cons cells `(:cons h t)` for relational decomposition (so
       `(conso a d l)` can split a list by head/tail without proper
       improper pairs); `mk-walk*` re-flattens cons cells back to native
       lists for clean reification output.
- **2026-05-07** — **Phase 3 done** (run + reification): `lib/minikanren/run.sx` (~28 lines).
  `reify`/`reify-s`/`reify-name` for canonical `_.N` rendering of unbound vars in
  left-to-right occurrence order; `run*` / `run` / `run-n` defmacros. 18 new tests
  in `tests/run.sx`, including the **first classic miniKanren tests green**:
  `(run* q (== q 1))` → `(1)`; `(run* q (fresh (x y) (== q (list x y))))` →
  `((_.0 _.1))`. 128/128 cumulative.
- **2026-05-07** — **Phase 2 piece D + done** (`condu` / `onceo`): `lib/minikanren/condu.sx`.
  Both are commitment forms: `onceo` is `(stream-take 1 ...)`; `condu` walks clauses
  and commits the first one whose head produces an answer. 10 tests in `tests/condu.sx`,
  110/110 cumulative. Phase 2 complete — ready for Phase 3 (run + reification).
- **2026-05-07** — **Phase 2 piece C** (`conde`): `lib/minikanren/conde.sx` — single
  defmacro folding clauses through `mk-disj` with internal `mk-conj`. 9 tests in
  `tests/conde.sx`, 100/100 cumulative. Confirmed eager DFS ordering for ==-only
  streams; true interleaving is a Phase 4 concern (paused thunks under recursion).
- **2026-05-07** — **Phase 2 piece B** (`fresh`): `lib/minikanren/fresh.sx` (~10 lines).
  defmacro form for nice user-facing syntax + `call-fresh` for programmatic use.
  9 new tests in `tests/fresh.sx`, 91/91 cumulative.
- **2026-05-07** — **Phase 2 piece A** (streams + ==/conj/disj): `lib/minikanren/stream.sx`
  (mzero/unit/mk-mplus/mk-bind/stream-take, ~25 lines of code) + `lib/minikanren/goals.sx`
  (succeed/fail/==/==-check/conj2/disj2/mk-conj/mk-disj, ~30 lines). Found and noted
  a host-primitive name clash: `bind` is built in and silently shadows user defines —
  must use `mk-bind`/`mk-mplus` etc. throughout. 34 tests in `tests/goals.sx`,
  82/82 cumulative all green. fresh/conde/condu/onceo still pending.
- **2026-05-07** — **Phase 1 done**: `lib/minikanren/unify.sx` (53 lines, ~22 lines of actual code) +
  `lib/minikanren/tests/unify.sx` (48 tests, all green). Kit consumption: `walk-with`,
  `unify-with`, `occurs-with`, `extend`, `empty-subst`, `mk-var`, `is-var?`, `var-name`
  all supplied by `lib/guest/match.sx`. Local additions: a miniKanren-flavoured cfg
  (treats native SX lists as cons-pairs via `:ctor-head = :pair`, occurs-check off),
  `make-var` fresh-counter, deep `mk-walk*` (kit's `walk*` only recurses into `:ctor`
  form, not native lists), and `mk-unify` / `mk-unify-check` thin wrappers. The kit
  earns its keep ~3× over by line count — confirms lib-guest match kit is reusable
  for logic-language hosts as designed.