Number of <disp-formula> elements that appear in the document.
<!ELEMENT equation-count EMPTY >
This is an EMPTY element
...
<book-meta>
...
<abstract>...</abstract>
<counts>
<count count-type="contributors" count="3"/>
<fig-count count="5"/>
<table-count count="3"/>
<equation-count count="10"/>
<ref-count count="26"/>
<page-count count="6"/>
<word-count count="2847"/>
</counts>
</book-meta>
...