I can't say I'm 100% happy with the slicing notation. I played around with some other wacky things, but then settled on overloading operator() to get a concise syntax.

In principle, you use it similar to python--s(5,10) instead of s[5:10].

I would have maybe liked to end up with s[5,10], but the only way to overload operator[] like that is with a pair. Then you'd end up with s[{5,10}]. So I figured it would be best to stick with just s(5,10).