Context sensitive code highlighting.

[Robin . Fairey]

How hard would it be to implement context sensitive font lock
behaviour? I'm thinking of something that scans the code for an
argument list and then highlights subsequent occurences of the
argument names in a new colour. This requires adding new keywords to
the highlighter on the fly, determined by content in the file itself
instead of in a keyword list at the beginning of the process. I've
tweaked some other syntax highlighting modes, but I've no idea where
to start with this one. I'm not looking for a full solution, just a
pointer to where to start looking, or something I can read and modify
to my needs?