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If you need to do a see or see also to a term that is a secondary term, indexers commonly use see under/see also under. It would be useful to allow an attribute for this purpose (possibly the class attribute, which is currently not used on see or seealso). The only impact on stylesheets would be to add gentext entries for see under and see also under and then add minimal code to the auto.idx stylesheet (I know stylesheets aren't covered here, so this is just to show that the implementation impact is minimal). If you used the class attribute, here is an usage example:
<indexterm><primary>foo</primary><secondary>bar</secondary></indexterm>
...
<indexterm><primary>bar</primary><seealso class="under">foo</seealso></indexterm>

In this case, the seealso on bar points to the term foo, and says to look under foo for sub-headings named bar.