Whenever iterator design comes up, I always have to refer to munificent's excellent article: https://journal.stuffwithstuff.com/2013/01/13/iteration-inside-and-

Whenever iterator design comes up, I always have to refer to munificent's excellent article: https://journal.stuffwithstuff.com/2013/01/13/iteration-inside-and-out/ https://journal.stuffwithstuff.com/2013/01/13/iteration-insi... tldr: The gold standard is if your iterator syntax / syntaxes can support these two types of iteration: 1. Interleaving two sequences 2. In-order tree traversal