Disabling minor mode after setting imenu-generic-expression

[Heime <heimeborgia@protonmail.com>]

Have set 'imenu-generic-expression' for a buffer when it is in 'emacs-lisp-mode'
using '(eq major-mode'.  Customarily, it is the major mode sets the default value
to 'imenu-generic-expression'.  I want to save the original value, so that when 
I deactivate the minor mode, I reset 'imenu-generic-expression' to any previous
value before the minor mode changed it.

How can I do so ?

(defun tema-expr-elisp ()

(setq imenu-generic-expression
  `( ("defun"
        ,(concat "^\\s-*"
                 "(\\(defun\\|define-minor-mode\\)\\s-+"
                 "\\(\\(\\sw\\|\\s_\\)+\\)") 2) )) )

;;;###autoload
(define-minor-mode tema-minor-mode
  "DESC."
  :init-value nil
  :lighter " Tema"

  (if tema-minor-mode
      (progn
        (cond
          ((eq major-mode 'sh-mode) (tema-expr-sh))
          ((eq major-mode 'emacs-lisp-mode) (tema-expr-elisp))))
    (tema-disable-function) ))