This now works:
```lean
inductive Tree where | node : List Tree → Tree
mutual
def Tree.size : Tree → Nat
| node ts => list_size ts
def Tree.list_size : List Tree → Nat
| [] => 0
| t::ts => t.size + list_size ts
end
```
It is still out of scope to expect to be able to use nested recursion
(e.g. through `List.map` or `List.foldl`) here.
Depends on #4718.
---------
Co-authored-by: Tobias Grosser <tobias@grosser.es>
