[tei-council] [TEI-L] ptr definition

[Syd Bauman]

As there seemed to be no objection whatsoever, I've changed cRef= of
<ptr> from optional to required (which does not mean that it is
required, only that either it or target= is required, as the two are
in an <attList org="choice">.

> I'd call that corrigible. This is in the Schematron bit, right? I
> can't believe anyone really uses that yet anyway.....