(metas, debug flags, transformations, formats) except for label.
This description is used in --list-*. The description can use any of
the formatting markup of Format "@ " "@[",...
Transformations can also specify from which metas and labels they
depend, and add informations about how they are interpreted.
TODO:
- complete and correct the documentation
- when a transformation use Trans.on_meta, it should be possible to
add an interpretation of the metas in the documentation.
- recover a summary version of --list-* ?
- be able to export in latex?
