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(defun is-even (x) (if (= x 0) #t (is-odd (- x 1)))) (defun is-odd (x) (is-even (- x 1))) (assert (is-even 100)) (assert (is-even 10)) (assert (is-even 0)) (assert (is-odd 1)) (assert (is-even 2)) (defun cnt-down (x) (if (= x 0) #t (progn (cnt-down (- x 1))))) (cnt-down 100)