def lsub(refname, mailbox)
      synchronize do
        send_command("LSUB", refname, mailbox)
        return @responses.delete("LSUB")
      end
    end
Sends a LSUB command, and returns a subset of names from the set
of names that the user has declared as being "active" or
"subscribed".  +refname+ and +mailbox+ are interpreted as
for #list().
The return value is an array of +Net::IMAP::MailboxList+.