mk: arith-progo — arithmetic progression generation

(arith-progo start step len result): result is the list
(start, start+step, ..., start+(len-1)*step). Length 0 yields the
empty list. Negative steps and zero step are supported.

Useful for FD-style domain construction:
  (arith-progo 1 1 9 dom)  -> (1 2 3 4 5 6 7 8 9)

6 new tests, 538/538 cumulative.

lib/minikanren/intarith.sx

@@ -135,3 +135,17 @@
     (conde
       ((nullo l) (== n 0))
       ((fresh (a d n-rest) (conso a d l) (conde ((== a x) (counto x d n-rest) (pluso-i 1 n-rest n)) ((nafc (== a x)) (counto x d n))))))))
+
+(define
+  mk-arith-prog
+  (fn
+    (start step len)
+    (cond
+      ((= len 0) (list))
+      (:else (cons start (mk-arith-prog (+ start step) step (- len 1)))))))
+
+(define
+  arith-progo
+  (fn
+    (start step len result)
+    (project (start step len) (== result (mk-arith-prog start step len)))))