∀ {α : Type w} {m : Type w → Type w'} {β : Type w} [inst : SizeOf α] [inst_1 : (a : Type w) → SizeOf (m a)]
  [inst_2 : SizeOf β] (nextIdx n : ℕ) (inner : Std.IterM m β),
  sizeOf { nextIdx := nextIdx, n := n, inner := inner } = 1 + sizeOf nextIdx + sizeOf n + sizeOf inner