Insert an element at a given position into a list

	;; P21 (*) Insert an element at a given position into a list.
	;; Example:
	;; * (insert-at 'alfa '(a b c d) 2)
	;; (A ALFA B C D)
	(define (insert-at elm lst pos)
	(let loop ((lst lst) (pos pos) (acc '()))
	(if (or (= pos 1) (null? lst))
	(append (reverse acc) (cons elm lst))
	(loop (cdr lst) (- pos 1) (cons (car lst) acc)))))

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