'Return message digest (unique id) list.
If \'which\', result contains unique id for that message
in the form \'response mesgnum uid\', otherwise result is
the list [\'response\', [\'mesgnum uid\', ...], octets]'

def uidl(self, which=None):

    if (which is not None):
        return self._shortcmd(('UIDL    %s' % which))
    else:
        return self._longcmd('UIDL')
