So far we've only been talking about MIME and SGML. Neither MIME
tools nor SGML tools currently grok URI's. I see no reason to
introduce that much new technology just to allow a MIME message to
hold multiple SGML entities.
Besides: MIME already specifies a way to reference bodies outside
the message: message/external-body. e.g.:
Content-Type: multipart/x-sgml; boundary="cut-here";
document=id1
--cut-here
Content-ID: id1
Content-Type: text/x-sgml
<!DOCTYPE HTML [
<!ENTITY % htmldtd SYSTEM "id2">
&htmldtd;
]>
<html>...</html>
--cut-here
Content-ID: id2
Content-Type: message/external-body;
access-type="anon-ftp";
site="info.cern.ch";
directory="pub/www/doc";
name="HTML.dtd"
Content-Type: text/x-sgml;
--cut-here--
Dan