mk: eveno + oddo — Peano parity predicates

eveno: zero, or (s (s m)) when m is even.
oddo:  one, or (s (s m)) when m is odd.

Both run forward (predicate test on a Peano number) and backward
(enumerate even / odd numbers). The two are mutually exclusive — no
number satisfies both.

12 new tests, 359/359 cumulative.

lib/minikanren/peano.sx

@@ -26,6 +26,22 @@
 
 (define lto (fn (a b) (fresh (sa) (succ-of a sa) (lteo sa b))))
 
+(define
+  eveno
+  (fn
+    (n)
+    (conde
+      ((== n :z))
+      ((fresh (m) (== n (list :s (list :s m))) (eveno m))))))
+
+(define
+  oddo
+  (fn
+    (n)
+    (conde
+      ((== n (list :s :z)))
+      ((fresh (m) (== n (list :s (list :s m))) (oddo m))))))
+
 (define
   *o
   (fn