Elements that can be included along with data characters inside the content model of the <supplement> element.
<!ENTITY % supplement-elements "%simple-text; | %contrib-group.class; | %label.class; | %title.class;" >
| email | ext-link | uri | inline-supplementary-material | related-article | related-object | hr | bold | fixed-case | italic | monospace | overline | overline-start | overline-end | roman | sans-serif | sc | strike | underline | underline-start | underline-end | ruby | alternatives | inline-graphic | private-char | chem-struct | inline-formula | tex-math | mml:math | abbrev | index-term | index-term-range-end | milestone-end | milestone-start | named-content | styled-content | fn | target | xref | sub | sup | x | contrib-group | label | title