ISO entities are not function names...

Greetings!
   I'm trying to shift from the SGML to the XML version of docbook. 
I've installed docbook-xml-4.2 on my RedHat 7.3 system.  My test 
document is pretty simple:

------------------
<?xml version='1.0' standalone="no"?>

  <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
                     "/usr/share/sgml/docbook/docbook-xml-4.2/docbookx.dtd">
<book>
<title>The Clear Lake Database</title>
</book>
------------------

When I try to validate the document using nsgmls -s from emacs, I get a 
long list of errors complaining that

nsgmls:/usr/share/sgml/docbook/docbook-xml-4.2/docbookx.dtd:112:17:E: 
"X20AC" is not a function name
nsgmls:/usr/share/sgml/docbook/docbook-xml-4.2/ent/iso-amsa.ent:8:19:E: 
"X21B6" is not a function name
  and so on.

I'm finding the DTD, and then the entities files, but cannot get the DTD 
to parse.  Clearly I do not know what I am doing, but can some kind soul 
tell me what to do?  Many thanks!