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+; P12 Decode a run-length encoded list.
+; Given a run-length code list generated as specified in problem P11.
+; Construct its uncompressed version.
+
+(def a (list (list 4 "a") "b" (list 2 "c") (list 2 "a") "d" (list 4 "e")))
+
+(func extend (lst1 lst2)
+    (reduce append lst2 lst1))
+
+(func flatten (lst)
+    (reduce
+        (lambda (x y) (extend x y))
+        lst
+        (list)))
+
+(func decode (lst)
+    (flatten
+        (map
+            (lambda (item)
+                (if (eq? 1 (length item)) 
+                    (list item) 
+                    (rpt (first item) (last item)))) 
+            lst)))
+            
+(func rpt (cnt let)
+    (if (eq? cnt 1)
+        (list let)
+        (extend (list let) (rpt (- cnt 1) let))))
+
+; iterative
+(func rpt-iter (cnt let)
+    (def out (list))
+    (for-count cnt
+        (redef out (append out let)))
+    out)
+
+(print (->string (decode a)))