It's just a way of saying: " Outside the scope of this textbook". The similar phrase, frequent in textbooks: " Proof left as an exercise to the reader" was

It's just a way of saying: " Outside the scope of this textbook". The similar phrase, frequent in textbooks: " Proof left as an exercise to the reader" was no less annoying. But you can't cover everything and given that sets aren't important to this post, indeed covering them would be more confusing since the Javascript Set.prototype has things such as being finite, and strict equality. This article is not an introduction to the mandelbrot set, it's "how to draw the mandelbrot set in javascript". I read this as a programming blog post not a mathematics blog post and enjoyed it.