Trying to find an old article about tree marks and math

The article started without any math context. It was about marking (real) trees with circles. And to "undo" the marking you would draw another circle inside this circle and so on... this turned into math after some pages..

Google isn't any help, my choise of keywords (tree, marks, math) might be bad and misleading.

But I think I first found this article on HN some years ago, maybe someone remembers it?