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@rauschma what about this version?

```ts
type Tree<T> = [Tree<T>, T, Tree<T>]|null;
```

And if we would like to make a balanced tree for immutable containers, then we can define a BTree23:

https://github.com/functionalscript/functionalscript/blob/f6a135390e16299984041b302ca8430e056f8366/types/btree/types/module.f.cjs#L46