∀ {α : Type u} {β : α → Type v} {k : α} {v : β k} {l : Std.DTreeMap.Internal.Impl α β} {rk : α} {rv : β rk}
  {rl rr : Std.DTreeMap.Internal.Impl α β},
  rl.Balanced →
    (Std.DTreeMap.Internal.Impl.rotateL k v l rk rv rl rr).size =
      (Std.DTreeMap.Internal.Impl.bin k v l (Std.DTreeMap.Internal.Impl.bin rk rv rl rr)).size