def user_chat(line):
    """
        Parse a message sent by user to the game chat.

        Input:
        `line`      # A string describing message sent by user to the game
                    # chat. Starts with 'Chat' prefix and contains sender's
                    # callsign and a body of the message. Example:
                    # "Chat: user0: \\tmessage"

        Output:
        An object providing information about message and its sender if the
        input line was successfully parsed. Otherwise `None`.
        """
