We will make the following backwards-incompatible changes in V3.0:
The SpanEnd attribute on IndexTerm will be renamed StartRef
to indicate that it points to the content of the IndexTerm being spanned.
A Class attribute will be added to indicate whether the IndexTerm element
is the beginning point of a span (StartOfRange), the ending point (EndOfRange),
or a standalone index term (Singular).
FootnoteRef will be declared EMPTY
and its Mark attribute renamed Label.