Virtual member functions

• virtual int check_event(void) - checks to see if a text command has been entered at the console or is available from a file. If so, this reads in the string, and queues a `TextEvent' command (which is a subclass of Command) containing the string.
• virtual int act_on_command(int, Command *, int) - called after a text event command is executed. This will take the text command string in the text event, break it into tokens, and if the first word is understood this will call the proper callback function. If this first word is not understood, or the callback function returns an error code, this will print an error message.