there are the following procedures:
UnknownClass, GroupClass, ControlClass for the rtf-classes with the same name.
start_para for the text-class at the beginning of each paragraph, and TextClass elsewhere.
The procedure UnknownClass deals as the name implies with unknown rtf-control-words.
GroupClass handles with groups. ( Groups are built both in rtf and LaTeX with {})
In the procedure ControlClass a case statement calls some procedures depending on the major-number. In these procedures it is easy to add some more lines to make a better version of this filter, which handles more rtf-control-words than mine.
(Many keywords aren't dealt with, e.g. the two rtf-control-words \clshdng, \verscomm, I have added in the reader.)