[plt-scheme] Attaching compile time information to identifiers

I'd like to write two macros, f & g, with the following usage:

(f id e ...)
(g id)

Such that for the same identifier id, g knows (during expansion) how 
many e's were given in (f id e ...), ie. I'd like f to attach some 
information to id that g can subsequently look up.  How can I do this?

Thanks,
David