On Jan 17, 3:28 pm, mistersquid <mistersq...
> I would like to filter items by saying something like "all items whose
> tags do not contain 'foo' ". Maybe an operator like '!' or "-" which
> would be typed in the search field as "!foo" or "-foo".
This isn't quite what you're looking for, but you can use AppleScript
to select all items that don't have a certain tag:
tell application "Yojimbo" to reveal (every database item where (name
of every tag does not contain "foo"))
If that's useful, it could be polished up with a dialog to input the
search term, and perhaps create a collection to contain the results.