[Evolution] How does expression work in a filter?

[Rick DeNatale <rick denhaven2 homeip net>]

In the create vfolder dialog and in the "advanced" dialog from the
filter dropdown, there is an "expression" option.

I can't seem to find any documentation on this. What can I put in the
expression?

I'd like to be able to filter on some of the SpamAssassin tags, for
example, I'd like to make a vfolder which shows any e-mails which have
RCVD_IN_* in the X-Spam-Status header to get an idea of which blacklists
being considered by SA are generating false positive indications.  Is
this something which can be done, either in 1.4 or in 1.5?