Adding Exercises to the DTD

<!ENTITY % local.formal.class "|exercise">

<!ENTITY % docbook.dtd PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                       "http://docbook.org/xml/4.3/docbookx.dtd">
%docbook.dtd;

<!ELEMENT exercise %ho; (blockinfo?, (%formalobject.title.content;),
                        (%example.mix;)+)
                        %formal.exclusion;>

<!ATTLIST exercise
                role    CDATA                   #IMPLIED
                %common.attrib;
>