First briefly read (but don't specifically respond to) https://adamsmith.as/papers/fdg2012generation.pdf This sets the stage for puzzle generation in Refraction and relates the content generation tasks to some familiar algorithms like depth-first search.
Next read (and do respond to) https://adamsmith.as/papers/fdg2013_shortcuts.pdf This pinpoints a flaw the in the previous work (and almost any previous generation system that tried to ensure interactive properties). Your questions could ask about either paper, but this one is the focus for motivating the next lecture in the class.
On the second to last page, there is a strange code example involving predicates and function symbols not described elsewhere: rule, pos, atom, wlist, minimize, optimize, and hold. They refer to a meta-interpreter for ASP: an ASP system implemented in ASP. Unfortunately, every line of this critical metaC.lp file is obsolete since we moved past clingo-3. Clingo-5 has a "--output=reify" flag that is almost the same as clingo-3's "--reify", and the meta.lp, metaD.lp, and metaO.lp on which this project depends have also been updated for Clingo-5. The bold keywords have all been slightly renamed in the new version.