Avoid `List.flatten`, it is a buggy abstraction (for the very same reason as flattening record traversal). Prefer `List.append`. Or if you prefer, define a function that "flattens" only the first 2 levels of a `List<List<A>>` into a a `List<A>`:
```
fun {Flatten1 Xss}
  {FoldR Xss List.append nil}
end
```