EmptyStep = object
  nil

SingleStep = object
  consumed: bool

ZipStep[SA; SB] = Pair[SA, SB]

LimitStep[S; N] = object
  step: S
  count: N

SkipStep[S; N] = object
  step: S
  count: N

TakeWhileStep[S; T] = object
  step: S
  item: Optional[T]

func emptyStep(): EmptyStep {...}{.raises: [], tags: [].}

func singleStep(consumed: bool): SingleStep {...}{.raises: [], tags: [].}

func isConsumed(X: typedesc[SingleStep]): Lens[X, bool]

func zipStep[SA; SB](left: SA; right: SB): ZipStep[SA, SB]

func limitStep[S; N: SomeNatural](step: S; count: N): LimitStep[S, N]

func step[S; N: SomeNatural](X: typedesc[LimitStep[S, N]]): Lens[X, S]

func count[S; N: SomeNatural](X: typedesc[LimitStep[S, N]]): Lens[X, N]

func skipStep[S; N: SomeNatural](step: S; count: N): SkipStep[S, N]

func step[S; N: SomeNatural](X: typedesc[SkipStep[S, N]]): Lens[X, S]

func count[S; N: SomeNatural](X: typedesc[SkipStep[S, N]]): Lens[X, N]

func takeWhileStep[S; T](step: S; item: Optional[T]): TakeWhileStep[S, T]

func step[S; T](X: typedesc[TakeWhileStep[S, T]]): Lens[X, S]

func item[S; T](X: typedesc[TakeWhileStep[S, T]]): Lens[X, Optional[T]]