mk: phase 5A — conda, soft-cut without onceo

conda-try mirrors condu-try but on the chosen clause it (mk-bind
(head-goal s) (rest-conj)) — all head answers flow through. condu by
contrast applies rest-conj to (first peek), keeping only one head
answer.

7 new tests covering: first-non-failing-wins, skip-failing-head, all-fail,
no-clauses, the conda-vs-condu divergence (`(1 2)` vs `(1)`), rest-goals
running on every head answer, and the soft-cut no-fallthrough property.

169/169 cumulative.

lib/minikanren/conda.sx

@@ -0,0 +1,42 @@
+;; lib/minikanren/conda.sx — Phase 5 piece A: `conda`, the soft-cut.
+;;
+;;   (conda (g0 g ...) (h0 h ...) ...)
+;;     — first clause whose head g0 produces ANY answer wins; ALL of g0's
+;;       answers are then conj'd with the rest of that clause; later
+;;       clauses are NOT tried.
+;;     — differs from condu only in not wrapping g0 in onceo: condu
+;;       commits to the SINGLE first answer, conda lets the head's full
+;;       answer-set flow into the rest of the clause.
+;;       (Reasoned Schemer chapter 10; Byrd 5.3.)
+
+(define
+  conda-try
+  (fn
+    (clauses s)
+    (cond
+      ((empty? clauses) mzero)
+      (:else
+        (let
+          ((cl (first clauses)))
+          (let
+            ((head-goal (first cl)) (rest-goals (rest cl)))
+            (let
+              ((peek (stream-take 1 (head-goal s))))
+              (if
+                (empty? peek)
+                (conda-try (rest clauses) s)
+                (mk-bind (head-goal s) (mk-conj-list rest-goals))))))))))
+
+(defmacro
+  conda
+  (&rest clauses)
+  (quasiquote
+    (fn
+      (s)
+      (conda-try
+        (list
+          (splice-unquote
+            (map
+              (fn (cl) (quasiquote (list (splice-unquote cl))))
+              clauses)))
+        s))))

lib/minikanren/tests/conda.sx

@@ -0,0 +1,75 @@
+;; lib/minikanren/tests/conda.sx — Phase 5 piece A tests for `conda`.
+
+;; --- conda commits to first non-failing head, keeps ALL its answers ---
+
+(mk-test
+  "conda-first-clause-keeps-all"
+  (run*
+    q
+    (conda
+      ((mk-disj (== q 1) (== q 2)))
+      ((== q 100))))
+  (list 1 2))
+
+(mk-test
+  "conda-skips-failing-head"
+  (run*
+    q
+    (conda
+      ((== 1 2))
+      ((mk-disj (== q 10) (== q 20)))))
+  (list 10 20))
+
+(mk-test
+  "conda-all-fail"
+  (run*
+    q
+    (conda ((== 1 2)) ((== 3 4))))
+  (list))
+
+(mk-test "conda-no-clauses" (run* q (conda)) (list))
+
+;; --- conda DIFFERS from condu: conda keeps all head answers ---
+
+(mk-test
+  "conda-vs-condu-divergence"
+  (list
+    (run*
+      q
+      (conda
+        ((mk-disj (== q 1) (== q 2)))
+        ((== q 100))))
+    (run*
+      q
+      (condu
+        ((mk-disj (== q 1) (== q 2)))
+        ((== q 100)))))
+  (list (list 1 2) (list 1)))
+
+;; --- conda head's rest-goals run on every head answer ---
+
+(mk-test
+  "conda-rest-goals-run-on-all-answers"
+  (run*
+    q
+    (fresh
+      (x r)
+      (conda
+        ((mk-disj (== x 1) (== x 2))
+          (== r (list :tag x))))
+      (== q r)))
+  (list (list :tag 1) (list :tag 2)))
+
+;; --- if rest-goals fail on a head answer, that head answer is filtered;
+;;     the clause does not fall through to next clauses (per soft-cut). ---
+
+(mk-test
+  "conda-rest-fails-no-fallthrough"
+  (run*
+    q
+    (conda
+      ((mk-disj (== q 1) (== q 2)) (== q 99))
+      ((== q 200))))
+  (list))
+
+(mk-tests-run!)

plans/minikanren-on-sx.md

@@ -124,8 +124,9 @@ Key semantic mappings:
       escapes to ground values for arithmetic or string ops
 - [ ] `matche` — pattern matching over logic terms (extension from core.logic)
       `(matche l ((head . tail) goal) (() goal))`
-- [ ] `conda` — soft-cut disjunction (like Prolog `->`)
-- [ ] `condu` — committed choice (already in phase 2; refine semantics here)
+- [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)
 - [ ] `nafc` — negation as finite failure with constraint
 - [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`
 
@@ -155,6 +156,10 @@ _(none yet)_
 
 _Newest first._
 
+- **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).