Code style guidance for new structure

Hi there.

I'm working on a functional AA tree, hoping to add it to this repository once tested, but I ran into a problem related to code style when implementing the IEnumerable interface.

Here is the file: https://github.com/hummy123/Functional-AA-Tree/blob/main/src/AaTree.fs .

The type at the top uses the module's (which comes after the type) toList function, which is enabled by the recursive namespace at the top.

However, this isn't best practice I feel (I like the "onion architecture" style where no piece at the top depends on any code written below), but the other choice I had was to code toList manually without using the fold function defined in the module, which I didn't like either.

I would have liked to add the toList definition and the namespaces at the end of the file where "type AaTree with..." is, but then I get a compile error FS0909 because an interface must be implemented at the start of the type definition. So I'm not sure what to do to contribute in a way that fits this repositories code style (haven't seen "namespace rec" on other files here).

Start with src/AaTree.fs, focusing on the AaTree type, its IEnumerable implementation, the recursive namespace, and the module's toList and fold functions. Review the existing project structure for comparable patterns; done means a maintainer-approved approach that compiles and fits the repository's code style.