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Hampshire Nanny - why not
My understanding is that would find all books that don't have a tag containing the string «source:»
It would exclude books that have a tag with «source:» in the middle or end of a tag, but do such tags actually exist in your library, e.g.
Code:
«tomato source: bottle»
«python source:»
«source:»
You can find the last with a search for
BR