∀ {α α₂ β : Type w} {m : Type w → Type w'} [inst : Std.Iterator α m (Std.IterM m β)] [inst_1 : Std.Iterator α₂ m β]
  {it₁ : Std.IterM m (Std.IterM m β)} {it₂ it₂' : Std.IterM m β} {b : β},
  it₂.IsPlausibleStep (Std.IterStep.yield it₂' b) →
    Std.Iterators.Types.Flatten.IsPlausibleStep { internalState := { it₁ := it₁, it₂ := some it₂ } }
      (Std.IterStep.yield { internalState := { it₁ := it₁, it₂ := some it₂' } } b)