Re: complex positioning problem

I'd try doing this by first processing the entire document expanding my citations into an (ad-hoc, locally-namespaced) markup format that provides each citation or footnote reference with whatever information it needs apart from the first/subsequent rule (that is, as if they were thence to be rendered all alike irrespective of their positions in the final list), and then in a second pass introduce the Ibid/Idem, op.cit., etc. based on their positions relative to one another. The rest of the second pass could be an identity transform otherwise, assuming there's nothing else it is useful for it to do.