rewrite [List.get_append_left as.data bs.data h]
α : Type u
i : Nat
as bs : Array α
h : i < size as
h' : i < size (as ++ bs)
⊢ List.get (as.data ++ bs.data)
      { val := { val := i, isLt := h' }.val,
        isLt := (_ : { val := i, isLt := h' }.val < List.length (as.data ++ bs.data)) } =
    as[i]
α : Type u
i : Nat
as bs : Array α
h : i < size as
h' : i < size (as ++ bs)
⊢ List.get as.data { val := i, isLt := h } = as[i]