How to search for unset boolean property

Hi all:

Boolean properties in Roundup can have three values: true, false and unset.

For link/multilink we can find unset properties by searching for '-1'.

For searching a boolean property, 0/1 finds false/true values. But how
to find unset?

Anybody with an answer or ideas how to do this?

Thanks.

--
				-- rouilj
John Rouillard
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My employers don't acknowledge my existence much less my opinions.