def examine(mailbox)
      synchronize do
        @responses.clear
        send_command("EXAMINE", mailbox)
      end
    end
Sends a EXAMINE command to select a +mailbox+ so that messages
in the +mailbox+ can be accessed.  Behaves the same as #select(),
except that the selected +mailbox+ is identified as read-only.

A Net::IMAP::NoResponseError is raised if the mailbox does not
exist or is for some reason non-examinable.