BITS 2.0
( ( object-id )*, label?, title?, subtitle*, alt-title*, ( address | alternatives
| answer | answer-set | array | boxed-text | chem-struct-wrap | code | explanation
| fig | fig-group | graphic | media | name-address-wrap | preformat | question | question-wrap
| question-wrap-group | 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 | x )*, ( sec
)* )
BITS 2.1
NO CHANGE IN CONTENT MODEL