lib/symbols for the commands supported, of course.
- Space: Needs checking.
- SpecialChar: Needs checking.
- - Split
+ - Split: See lib/symbols, for split insets.
- Substack: This is a stack of however many cells, all in a smaller style.
Probably do something with <mover>, again.
- Tabular: This is more or less a text-like table in math. Probably output it