DOCBOOK: can't grok nsgmls complaint

["A.R. (Tom) Peters" <tpeters@xs4all.nl>]

I am writing a book is DocBook-XML.  Its bare bones look like this:

# GemBook.xml
<?xml version='1.0'?>

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
                      "http://www.oasis-open.org/docbook/xml/4.0/docbookx.dtd"; [
<!ENTITY PartA SYSTEM "A.xml">
]>
<book lang="nl">
<title>Cursus Gemmologie</title>
&PartA;
</book>

# A.xml
<chapter>
<title>Theoretische Gemmologie</title>
</chapter>


validating it with nsgmls gives this complaint:

$ nsgmls -s -wxml /usr/share/sgml/declaration/xml.dcl GemBook.xml
nsgmls:/usr/share/sgml/declaration/xml.dcl:1:W: SGML declaration was not
implied
nsgmls:A.xml:3:9:E: end tag for "chapter" which is not finished


How so?

N.B. adding more tags in the chapter yields similar complaints.

BTW I am using Debian, and xml.dcl comes from the sgml-data package
(v1.5.5), and it seems to be a regular XML definition from 1993.

--
#>!$!%(@^%#%*(&(#@#*$^@^$##*#@&(%)@**$!(&!^(#((#&%!)%*@)(&$($$%(@#)&*!^$)^@*^@)

	Tom "thriving on chaos" Peters
		NL-1062 KD nr 149	tel.    +31-204080204
 			Amsterdam	e-mail  tpeters@xs4all.nl