diff --git a/persistent-outline.el b/persistent-outline.el
index 9e5d052..1a0bb95 100644
--- a/persistent-outline.el
+++ b/persistent-outline.el
@@ -4,6 +4,7 @@
 
 ;; Author: Tom Willemse <tom@ryuslash.org>
 ;; Keywords: convenience, outlines
+;; Version: 0.1.0
 
 ;; This program is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by
@@ -47,8 +48,11 @@
   :group 'persistent-outline
   :type 'directory)
 
-(defclass perso-file-local-storage () ())
-(defclass perso-filesystem-storage () ())
+(defclass perso-file-local-storage () ()
+  :documentation
+  "A storage back-end that saves the state in a file-local variable.")
+(defclass perso-filesystem-storage () ()
+  "A storage back-end that saves the state in a dedicated file.")
 
 (defvar perso-outline)
 
@@ -134,6 +138,9 @@ functions for creating backup and autosave filenames."
     (unless (file-exists-p directory)
       (make-directory directory :parents))))
 
+(defgeneric perso-save-to-storage (storage outline)
+  "Tell the storage back-end to save the current outline state.")
+
 (defmethod perso-save-to-storage ((storage perso-file-local-storage)
                                   outline)
   "Save the current states to a file-local variable."
@@ -150,6 +157,9 @@ functions for creating backup and autosave filenames."
       (print outline (current-buffer))
       (write-file filename))))
 
+(defgeneric perso-load-from-storage (storage)
+  "Tell the storage back-end to load the saved outline state.")
+
 (defmethod perso-load-from-storage ((storage perso-file-local-storage))
   "Load saved states from a file-local variable."
   (when (boundp (make-local-variable 'perso-outline))