* `structure` and `inductive` commands
  * [#5842](https://github.com/leanprover/lean4/pull/5842) and [#5783](https://github.com/leanprover/lean4/pull/5783) implement a feature where the `structure` command can now define recursive inductive types:
    ```lean
    structure Tree where
      n : Nat
      children : Fin n → Tree

    def Tree.size : Tree → Nat
      | {n, children} => Id.run do
        let mut s := 0
        for h : i in [0 : n] do
          s := s + (children ⟨i, h.2⟩).size
        pure s
    ```
  * [#5814](https://github.com/leanprover/lean4/pull/5814) 