Persistent undo across editing sessions

[Marco]

Hi all!

I am new to the list and new to emacs.

I'd like to have this feature and I don't understand how to implement
it: I'd like to be able to have the undo history of one file kept
between different editing sessions (possibly then using undo-tree
which I found around).

Is this possible? (I guess it is: my impression is that *everything*
is possible in emacs)
Does someone have an idea on how to do it?

Thanks,

marco

PS
RTFMs are welcome, provided that you give me a link on where to read ;)