def mission_status(lines):
    """
        Parse information about mission's status.

        Input:
        `lines`     # A sequence of strings providing information about
                    # mission's status. They tell whether mission is loaded,
                    # is it playing or it is not loaded. Input strings are
                    # server's response for such commands as 'mission',
                    # 'mission LOAD', 'mission BEGIN'.

        Output:
        An object which describes current mission's status if the input line
        was successfully parsed. Otherwise `None`.
        """
