[imp] How is Virtual Trash set up in IMP 5?

[francis picabia]

Looking through the preferences and config UI, can't see how Virtual
Trash is enabled.   I've seen how to set it as the trash, but that
won't create or enable it. There is an IMAP error if I only point the
preference to Virtual Trash.

This is all I have to go by, and I don't understand what other values
it could be assigned :

$_prefs['vfolder'] = array(
    // By default, Virtual Inbox is enabled and Virtual Trash is disabled.
    // 'value' => serialize(array())
    'value' => 'a:0:{}'
);