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Footnotes and Footnote References
Footnote contains one or more object-level elements from the %footnote.mix; mixture. It may not contain a Footnote element.
FootnoteRef is an EMPTY element. It has a Linkend attribute, and an optional Mark attribute (which will be changed to Label in V3.0) which provides symbols to be used to point to footnote text; the default is whatever was used in the original footnote being referenced.