transform-at.scm

(use-modules (srfi srfi-1)
             (rnrs))

(define (transform-at l i val)
  (let-values (((left right) (split-at l i)))
    (append left (cons val (cdr right)))))

(display (transform-at '(1 2 3) 1 5)) ;=> '(1 5 3)

Makes sense, thanks!