One entry in a structural Table of Contents.
The following, in order:
<label> Label of a Figure, Reference, Etc., zero or one
<title> Title, zero or more
<subtitle> Article Subtitle, zero or more
Optionally any one of:
<ext-link> External Link
<nav-pointer> Navigation Pointer
<nav-pointer-group> Navigation Pointer Group
<toc-entry> Table of Contents Entry, zero or more