Some Schemes - most notably Racket, which I use the most - allow for nested defines. Your code actually is a valid Racket code after polishing the differences i

Some Schemes - most notably Racket, which I use the most - allow for nested defines. Your code actually is a valid Racket code after polishing the differences in naming and syntax a bit: #lang racket (define-syntax-rule (defun name (arg ...) . body) (define (name arg ...) . body)) (define nil null) (define t #t) (defun ab (l) ;; "I'm too lazy to reverse the result back." (defun _ (l a b f) (if (null? l) (if f (list a b) (list b a)) (_ (cdr l) b (cons (car l) a) (not f)))) (_ l nil nil t)) (ab '()) (ab '(1)) (ab '(1 2)) (ab '(1 2 3 4 5 6 7 8 9 10)) ;; ((9 7 5 3 1) (10 8 6 4 2)) (ab '(1 2 3 4 5 6 7 8 9 10 11)) ;; ((11 9 7 5 3 1) (10 8 6 4 2)) ...and you don't need to worry about recursion limits thanks to mandatory TCO.