I remember him from Fish and multidimensional arrays.

https://link.springer.com/article/10.1007/s100090050037

Much better intro article about tree calculus here, vs the actual site: https://olydis.medium.com/a-visual-introduction-to-tree-calc...

Extensive discussion (202 comments) about 15 months ago: https://news.ycombinator.com/item?id=42373437

That makes me think of the Inca's quipus.

I'm not used to math things being promoted like this (not to suggest that's a bad thing at all!). Can someone offer some context please.