I think it would mean that a phrase/sentence is being modified by a phrase containing the word of , and that this "of phrase" comes after it. Can you give some context? The examples you wrote don't contain the word of at all.
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Laboriouspost - modification by an of phrasePre-modifiers go before the thing they modify; post-modifiers go after the thing they modify.
ElanguestThe examples you wrote don't contain the word of at all.
Laborious In my second example "Total number of signatures... .", isn't 'of' pre - modifying the 'signatures'?No. Prepositions don't modify. They are called the "head" of "preposition phrases".
LaboriousCould you give me an example or two of pre - modification (of something, for e.g. a 'noun') by an "of - phrase", please?