[plt-scheme] optional parameters?

Hi again,

I'm using prometheus (
http://planet.plt-scheme.org/package-source/daedalus/prometheus.plt/1/0/doc.txt
)for creating objects, and it's working fine. However, I have a
method, which I would like to give an optional argument. How can I
make an argument optional, and how do I specify a default value for
when it isn't supplied? Is it even possible, or should I use a
different strategy?

-- 
troels