[Evolution] Use of Scheme in Search Folder filters

[David Sumbler <david aeolia co uk>]

I am trying to create a filter which, as one condition, excludes
messages with a value > 5.0 in the "X-Spam-Score" header.  

So far as I can see, I would need to use the "Expression" option to do
this.  The manual says "Match a message according to an expression you
write in the Scheme language used to define filters in Evolution",
which seems a bit circular.

I am familiar with Scheme, but there is no indication here about how
the Scheme expression should be formed.  For instance, what is required
might be something such as "(if (> 5.0 X-Spam-Score) #f)", but that one
certainly doesn't work, and there are too many other plausible
possibilities simply to use trial and error.

Can somebody point me to an explanation of how such Scheme expressions
in Evolution should be formed, or perhaps show me how (and whether)
this one can be made to work?

David