nXML "No completions available"

[Bob Bernstein <poobah@ruptured-duck.com>]

I am in my new-ish:
Linux debian.localdomain 4.9.0-9-amd64 #1 SMP Debian 
4.9.168-1+deb9u3 (2019-06-16) x86_64 GNU/Linux

I seek to edit a DocBook XML file in emacs23. The file 
begins:

<?xml version='1.0' ?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" 
"/usr/share/sgml/docbook/dtd/xml/4.4/docbookx.dtd" []>

Does that seem okay? However, trying out autocompletion 
with Esc-TAB gives me the error message noted in my 
Subject: "no completions available."

I created the small file many years ago and am able to use 
xsltproc and an xsl style sheet to produce a suitable pdf. 
Now I want to edit it. I remember virtually nothing of how 
to actually _use_ nXML in emacs, that is, keystrokes. 
(How, for example can I bring up a list of valid tags for 
insertion at a given point in the file?)

Thank you

-- 
Never ascribe to malice what can be explained by stupidity.