Function get_learning_objectives 

pub fn get_learning_objectives(blocks: &[GutenbergBlock]) -> Option<String>

The learning objectives declared anywhere in blocks, one per line, or None when the page declares none.

Objectives are authored as list items inside a learning-objectives block, so the lines are the objectives themselves. Whether they cover the page, the chapter or the course depends on which page the blocks came from, which is the caller’s to know.