@forIn.loop.eq_1 : ∀ {m : Type u_1 → Type u_2} {β : Type u_1} [inst : Monad m] (f : UInt8 → β → m (ForInStep β))
  (arr : ByteArray) (off _end i : Nat) (b : β),
  forIn.loop f arr off _end i b =
    if h : i < _end then do
      let __do_lift ← f (arr.get! i) b
      match __do_lift with
        | ForInStep.done b => pure b
        | ForInStep.yield b =>
          have this := ⋯;
          forIn.loop f arr off _end (i + 1) b
    else pure b