(require "lists.BRU" "logic.BRU" "evenness-combinators.BRU")
[(mutual-even-odd? (Y2-choose (cons (\f
           (\n
             (if (zero? n)
                 #t
                 (f (1- n)))))
         (\f
           (\n
             (if (zero? n)
                 #f
                 (f (1- n))))))))]