[Zope] Not grocking RESQUEST.set syntax

[Noah]

I'm still not grocking the syntax for setting REQUEST fields.
I want to set a REQUEST field "filename" to be the value returned
by a ZSQL call. I want something like this (pseudo-code of what I want):

<dtml-let FOO=MyFavoiteZSQLMethod>
<dtml-call "REQUEST.set('filename', FOO)"> 
<dtml-call some_other_thing_that_uses_filename>
</dtml-let>

That does not work because FOO gets set to something like:
<Shared.DC.ZRDB.Results.Results instance at 7760b0>
I tried sticking this inside <dtml-in><dtml-in> to get
at the sequence-item, but I just ended up getting confused.

Yours,
Noah