Chris Rowley writes:
 > ... mostly about normalisation and concrete syntax.
 >
 > He did not mention the lack of extensibility: was this deliberate?
 >
 > It is tortuously difficult in SGML to have bothe extendabilty and
 > portability; it is therefore impossible to have extendability in XML
 > (correct?).
 >
"tortuously difficult"? but a standard feature of two well known DTDs,
TEI and Docbook. What works there works in XML versions, of
course. It makes your documents less portable, if by that you mean
that processing programs have to be very defensive, but its a
perfectly plausible way to work.

Sebastian