Suggestion: add list of all learning outcomes in each book's frontmatter

[StevenClontz]

This was suggested to me from a friend evaluating our library - they wanted a bird's-eye view of what we cover in each book.

[siwelwerd]

Hmmm, right now objectives are in individual files, and are displayed in an <ol> within an <objectives>. I guess just using <list-of elements="objectives"/> will work, but we might want to rethink the introduction text in the objectives.

This also might be good to list on the website, but probably less automatically.