＆＃39; lambda做什么？

Question

这是emacs的源代码。 'lambda在(eq transient-mark-mode 'lambda)中做了什么？

(defun handle-shift-selection ()
  (cond ((and shift-select-mode this-command-keys-shift-translated)
     (unless (and mark-active
          (eq (car-safe transient-mark-mode) 'only))
   (setq-local transient-mark-mode
                   (cons 'only
                         (unless (eq transient-mark-mode 'lambda)
                           transient-mark-mode)))
       (push-mark nil nil t)))
    ((eq (car-safe transient-mark-mode) 'only)
     (setq transient-mark-mode (cdr transient-mark-mode))
     (if (eq transient-mark-mode (default-value 'transient-mark-mode))
         (kill-local-variable 'transient-mark-mode))
     (deactivate-mark))))