Compute the inherited value of ttm:role

[nigelmegitt]

Downstream processing may require access to the applicable value of the ttm:role attribute on a content element. Semantically, the value of this metadata is effectively inherited.

For example:

<div ttm:role="caption">
   <p xml:id="pCaption">A caption</p>
   <p ttm:role="sound" xml:id="pSound">A sound</p>
</div>

Being able to compare the role of the two p elements here would be helpful for semantic processing, without having to climb the tree every time.