Some more ISLisp->CL examples

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+;; Ported from https://rosettacode.org/wiki/Assertions_in_design_by_contract#Eiffel
+;; Arguably not *quite* design-by-contract, but very close in vanilla CL
+(defun average-of-absolutes (values)
+  (declare (list values))
+  (assert (> (length values) 0))
+  (let ((res (/ (reduce #'+ (mapcar #'abs values)) (length values))))
+    (assert (>= res 0))
+    (the fixnum res)))
+;; (average-of-absolutes '(1 3))
+;; (average-of-absolutes '())