A-Z parts
Structure A-Z reference content so that you capture each letter set in its own part within the mainText (if it is a complete title). Set the role attribute to the letter that the set belongs to.
//part/@role
Example
<mainText>
<part role="A"><e id="acref-9780195123456-e-0355" doi="10.1093/acref/9780195123456.013.0355">
<headwordGroup><!-- Content skipped for brevity --></headwordGroup></e><e id="acref-9780195123456-e-0356" doi="10.1093/acref/9780195123456.013.0356">
<headwordGroup><!-- Content skipped for brevity --></headwordGroup></e>
</part>
<part role="B"><e id="acref-9780195123456-e-0357" doi="10.1093/acref/9780195123456.013.0357">
<headwordGroup><!-- Content skipped for brevity --></headwordGroup></e><e id="acref-9780195123456-e-0358" doi="10.1093/acref/9780195123456.013.0358">
<headwordGroup><!-- Content skipped for brevity --></headwordGroup></e>
</part>
</mainText>