Replaces the ordinalth value of vlist with value. If span > 1, replaces ordinal to ordinal+span-1 values starting at ordinal.
(define (list-put vlist ordinal value #!optional (span 1))
  ;; Replace a specific member of a list
  (let loop ((result vlist) (count span) (k ordinal))
    (if (equal? count 0)
	result
	(let ((head (list-head result (- k 1)))
	      (tail (list-tail result k)))
	  (loop (append head (list value) tail) (- count 1) (+ k 1))))))