McAfee IDS signature writing

Hello list

Does anyone have any experience with writing signatures for McAfee IPS systems? 
 It's a bit frustrating compared to a system like Snort, because the 
vendor-supplied sigs are "secret sauce".  I can't just look in there for 
examples similar to what I'm trying to achieve.

What I'm after in this case should in principle be relatively simple - I want 
to catch certain function calls in an HTTP response, but only in the context of 
a javascript block.  I'd like to avoid tripping the signatures if the same 
strings come up in the regular text of a page, e.g. a in a mailing list posting 
describing an IDS signature or a browser vulnerability...

Regards
Mark

PS - kindly cc me on replies, as I'm not subscribed to the list

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