[Self-response] The draft version (linked by edmccard in another comment) now has a brief explanation in the margin note that these are references to programmi

[Self-response] The draft version (linked by edmccard in another comment) now has a brief explanation in the margin note that these are references to programming books: http://www.ccs.neu.edu/home/matthias/HtDP2e/Draft/index.html http://www.ccs.neu.edu/home/matthias/HtDP2e/Draft/index.html