def user_left(line):
    """
        Parse information about disconnected user.

        Input:
        `line`      # A string describing disconnected user. Example:
                    # "socketConnection with 192.168.1.2:21000 on channel 0 lost.  Reason: "

        Output:
        An object providing information about disconnected user if the input
        line was successfully parsed. Otherwise `None`.
        """
