mk: sumo + producto — fold list to integer

Sum and product over a list of ground integers via fold + intarith.
Empty list yields the identity (0 for sum, 1 for product). Recurse
combines the head with the recursively-computed tail value via
pluso-i / *o-i.

9 new tests, 481/481 cumulative.

lib/minikanren/intarith.sx

@@ -93,3 +93,19 @@
     (conde
       ((fresh (a) (== l (list a)) (== m a)))
       ((fresh (a d rest-max) (conso a d l) (maxo d rest-max) (conde ((lteo-i rest-max a) (== m a)) ((lto-i a rest-max) (== m rest-max))))))))
+
+(define
+  sumo
+  (fn
+    (l total)
+    (conde
+      ((nullo l) (== total 0))
+      ((fresh (a d rest-sum) (conso a d l) (sumo d rest-sum) (pluso-i a rest-sum total))))))
+
+(define
+  producto
+  (fn
+    (l total)
+    (conde
+      ((nullo l) (== total 1))
+      ((fresh (a d rest-prod) (conso a d l) (producto d rest-prod) (*o-i a rest-prod total))))))