Specialization of syntax diagram

["Paul Prescod" <paul.prescod@blastradius.com>]

pr-d/syntaxdiagram is a specialization of topic/fig. It has the
following elements in its content model: pr-d/fragref, pr-d/synnote,
pr-d/synnoteref.

These are specializations of xref, footnote and xref respectively.
topic/fig cannot contain footnote or xref (directly). Is this a bug in
DITA?

I discovered this discrepency while testing a prototype specialization
validator plugin for XMetaL.

 Paul Prescod