emacs toggle-fullscreen incl. f11 binding

.emacs

(defun toggle-fullscreen (&optional f)
  (interactive)
  (let ((current-value (frame-parameter nil 'fullscreen)))
       (set-frame-parameter nil 'fullscreen
                            (if (equal 'fullboth current-value)
                                (if (boundp 'old-fullscreen) old-fullscreen nil)
                                (progn (setq old-fullscreen current-value)
                                       'fullboth)))))

(global-set-key [f11] 'toggle-fullscreen)