States & Query
This section is outdated. It may be still useful, but it is strongly recommended to study new context system (using the reference).
Typst tries to be a pure language as much as possible.
That means, a function can't change anything outside of it. That also means, if you call function, the result should be always the same.
Unfortunately, our world (and therefore our documents) isn't pure. If you create a heading №2, you want the next to be number three.
That section will guide you to using impure Typst. Don't overuse it!