User blog comment:Plain'N'Simple/Using triangular numbers to create a Conway-Arrow Level Notation (probably not what you'd expect)/@comment-35434595-20191025155632/@comment-35470197-20191027010114

> I know the formulation, but never knew that it was Hausdorff who invented it.

Oops, I mistook. It is Kuratowski's convention. Hausdorff's convention is given by (x,y) = {{x,1},{y,2}}. Sorry for the inaccuracy.