def user_joined(line):
    """
        Parse information about joined user.

        Input:
        `line`      # A string describing new server connection. Example:
                    # "socket channel '0', ip 192.168.1.2:21000, user0, is complete created."

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