Functions to build ToCs

[JimMadge]

I'm trying to build a MyST plugin which allows you to curate pages/sections of a book into new ToCs, which could be used as syllabi, or sub-books.
This is different from collecting pages with the same tag, as you have the order and nesting of a ToC.

My current idea is to have a directive, where the body uses the same format at the ToC defined in myst.yml, like,

:::{curation} my favourites
- file: animals/cats.md
- file: cinemas/prince_charles.md
- file: places/lundy.md
:::

which will end up in AST something like,

- type: block
  children:
    - type: toc
      children:
        - type: heading
          level: 2
          value: My Favourites
        - type: list
          children:
            - type: ListItem
              children:
                - type: link
                  url: /animals/cats
                  children:
                    - type: text
                      value: Cats
...

as when you use the {toc} directive.
To make this consistent with other ToCs and avoid duplication, it would be great to use functions/transformations in the MyST packages which do this.

I haven't found quite what I'm looking for.

The {toc} directive and transformation don't let you write a ToC explicitly.
I found a ToC validations function which is helpful.
However, what I think is the function to process the toc item in myst.yml looks like it takes arguments about the project.

Is there a function like structured_data -> ToC_AST in MyST?
Maybe there is a better way to go about this?