Hey Bob,

I suspect that Fong and Spivak used TikZ to make these string diagrams.

Here’s some links on how various people do it:

- https://tex.stackexchange.com/a/264599

- http://www.ryancreich.info/index.php?page=latex&subpage=string_diagrams

- https://golem.ph.utexas.edu/category/2018/01/freetikz.html