There is a small conflict case,
<ref name="a">book</ref> <ref extends="a" name="b">page 2</ref> <ref extends="a" name="c">page 3</ref> <ref name="b" /> <ref name="c" /> <references />
For comparison, a similar snippet without book referencing will render as,
<ref name="a">book</ref> <ref name="b">page 2</ref> <ref name="c">page 3</ref> <ref name="b" /> <ref name="c" /> <references />
The references section backlinks for reuse footnotes are wrong, and need design discussion. They should probably be "1.1.1" and "1.1.2" ?
We also need to consider how alternative backlink symbols are rendered, although this is probably correct with default behavior.
