Content model for the Structural Index <index> element.
<!ENTITY % index-model "(title-group?, (%para-level-minus-x;)*, (%index-entry.class; | %index-div.class; | %x.class;)+ )" >
(title-group?, (address | alternatives | answer | answer-set | array | boxed-text | chem-struct-wrap | code | fig | fig-group | graphic | media | preformat | question | question-wrap | supplementary-material | table-wrap | table-wrap-group | disp-formula | disp-formula-group | def-list | list | tex-math | mml:math | p | related-article | related-object | ack | disp-quote | speech | statement | verse-group)*, (index-entry | index-div | x)+ )