The OP doesn't want encompassing, they want the following example from the tutorial on the front page:

    type vec (a:Type) : nat -> Type =
      | Nil : vec a 0
      | Cons : #n:nat -> hd:a -> tl:vec a n -> vec a (n + 1)
    
    let rec append #a #n #m (v1:vec a n) (v2:vec a m)
      : vec a (n + m)
      = match v1 with
        | Nil -> v2
        | Cons hd tl -> Cons hd (append tl v2)

This is a completely reasonable thing to want and expect.

Edit: For comparison, Rocq https://rocq-prover.org/ and Lean https://lean-lang.org/ both manage to do this.