Add all the A tasks

Task/Anonymous-recursion/Racket/anonymous-recursion-1.rkt

@@ -0,0 +1,18 @@
+#lang racket
+
+;; Natural -> Natural
+;; Calculate factorial
+(define (fact n)
+  (define (fact-helper n acc)
+    (if (= n 0)
+        acc
+        (fact-helper (sub1 n) (* n acc))))
+  (unless (exact-nonnegative-integer? n)
+    (raise-argument-error 'fact "natural" n))
+  (fact-helper n 1))
+
+;; Unit tests, works in v5.3 and newer
+(module+ test
+  (require rackunit)
+  (check-equal? (fact 0) 1)
+  (check-equal? (fact 5) 120))