Garbage collect and buffer-undo-list

[Yuan Fu]

I know the garbage collector trims buffer-undo-list. Does it collect the cons 
cell if there are other references to that object (other than buffer-undo-list)?

I tried this:

(setq last1 (last buffer-undo-list))
(garbage-collect)

After the garbage collection, the entry referred by last1 is not in 
buffer-undo-list anymore, but still accessible by last1. That seems to support 
my theory.

Yuan