iso 12083 math

I've put up two messages from Eric van Herwijnen in June of 1993 which
present and partially explain the math dtd fragment of ISO 12083.  This
is for reference by those who want to get a sense of the DTD.  A few
corrections were made recently, but they don't dramatically alter
the relation the DTD might have to our display-list format.

The URL to access is www.ams.org/html-math/erb/.  Userid and password
are "markup" and "math".

I'm still trying to find time to read and comment on Robert's posting
earlier this week on display-list format.