mk: phase 2C — conde, the canonical and-or sugar

conde.sx is a single defmacro: (conde (g1a g1b ...) (g2a g2b ...) ...) folds
to (mk-disj (mk-conj g1a g1b ...) (mk-conj g2a g2b ...) ...). 9 tests cover
single/multi-clause, mixed success/failure, conjunction inside clauses,
fresh+disj inside a clause, nesting, and all-fail / no-clauses.

100/100 cumulative.

lib/minikanren/conde.sx

@@ -0,0 +1,20 @@
+;; lib/minikanren/conde.sx — Phase 2 piece C: `conde`, the canonical
+;; miniKanren and-or form.
+;;
+;;   (conde (g1a g1b ...) (g2a g2b ...) ...)
+;;     ≡ (mk-disj (mk-conj g1a g1b ...)
+;;                (mk-conj g2a g2b ...) ...)
+;;
+;; Each clause is a list of goals, conj'd internally; clauses are disj'd
+;; among one another (interleaved via mk-mplus, so left-recursive
+;; relations don't starve the right-hand clauses).
+
+(defmacro
+  conde
+  (&rest clauses)
+  (quasiquote
+    (mk-disj
+      (splice-unquote
+        (map
+          (fn (clause) (quasiquote (mk-conj (splice-unquote clause))))
+          clauses)))))