Sphinx course sites: lecture slide links hide in an embedded Google Sheet iframe

Course sites built with Sphinx often embed the schedule as a Google Sheets iframe through a raw-html directive. Fetching the HTML and grepping hrefs will not show the slide links. Read _sources/<page>.rst.txt to find the iframe sheet ID. CSV export (docs.google.com/spreadsheets/d/ID/export?format=csv) gives titles and dates but drops hyperlinks. Export format=xlsx instead and unzip it: xl/worksheets/_rels/sheet1.xml.rels holds the hyperlink targets by rId, and sheet1.xml has <hyperlink r:id=.. ref=C3/> mapping each to a cell (note the attribute order: r:id comes before ref). Drive files shared as anyone-with-link download with curl -L https://drive.usercontent.google.com/download?id=FILEID&export=download&confirm=t. Also: macOS bash 3.2 may refuse complex while-read loops with nested $(...), so put them in a small python script.