DOCBOOK-APPS: DocBook, MathML and Delimiter GRPC

Hello everybody,
    I have a problem using MathML and XML DocBook V4.1.2. I've downloaded
the dbmathml.dtd file from www.oasis-open.org/docbook/, and MathML2 DTD from
the www.w3c.org website. Then I've installed them in the right way (I hope
so). But when I want to edit an XML file with the DOCTYPE

<?xml version="1.0"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.0beta3//EN"
"http://www.oasis-open.org/docbook/xml/mathml/1.0beta2/dbmathml.dtd">

using Emacs, an error message appears when parsing the DTD:

/usr/lib/xml/ent/iso-tech.ent line 1 col 33 entity equation.content
/usr/lib/xml/dbpoolx.mod line 3219 col 34 entity dbpool
/usr/lib/xml/docbookx.dtd line 91 col 8 entity docbook
/usr/lib/xml/dbmathml.dtd line 63 col 9 entity BOOK
/data/documentroot/SGML/tmp.xml line 2 col 73
Delimiter GRPC ()) expected; at: :math))

I've heard the problem could be in precompiled DTDs, but I don't use that
(the necessary DTD is parsed when a file is opened). So where's the problem?

Thanks
Tom