[plt-scheme] Warnings from macros?

This is for a homework assignment, but the assignment was to write a
parser for a grammar (in a language of our choice), and I decided to
write a parser generator as a scheme macro instead, (much more fun =D)
so this question is really outside the scope of the assignment.

I have a macro parser-from-grammar, and I want to flag detected
ambiguities based on overlaps between calculated first and follow
sets. I can call "error", but that will only give one error before
dying. Is there a way to emit the equivalent of compilation warnings
from a macro? It seems output from printf gets ignored.

    Henk