Returns the target in replace-list that matches in string at pos See also repl-substring-list?.
(define (repl-substring-list-target string replace-list pos)
  ;; Return the target that matches in a string
  (let loop ((list replace-list))
    (let ((target (car list))
	  (repl   (car (cdr list)))
	  (rest   (cdr (cdr list))))
      (if (repl-substring? string target pos)
	  target
	  (if (null? rest)
	      #f
	      (loop rest))))))