(defun compose-disable-all-followups (mode-map key-to-revert-l)
  (dolist (key-to-revert key-to-revert-l)
    (let ((key    (car   key-to-revert))
          (revert (nth 2 key-to-revert)))
      (define-key mode-map key revert) )))

(defun compose-followup-key (mode-map key-to-revert-l)
  (dolist (key-to-revert key-to-revert-l)
    (let ((key    (car   key-to-revert))
          (to     (nth 1 key-to-revert))
          (revert (nth 2 key-to-revert)))
      (define-key (symbol-value mode-map) key
        `(lambda ()
           (interactive)
           (insert ,to)
           (compose-disable-all-followups
            ,mode-map
            ',key-to-revert-l ))))))

(defun activate-semi-colon-compose-key ()
  (interactive)
  (compose-followup-key
   'text-mode-map
   '(("A" "Ä" self-insert-command)
     ("a" "ä" self-insert-command)
     ("O" "Ö" self-insert-command)
     ("o" "ö" self-insert-command)
     (":" "Å" self-insert-command)
     (";" "å" activate-semi-colon-compose-key) )))

(define-key text-mode-map ";" 'activate-semi-colon-compose-key)