optionally
fun <S, T : Any> SenderScope<S>.optionally(validatedDefault: ValidatedDefault<S, T>, parameter: StructureElement<S, Parameter<S, T>>): StructureElement<S, SignatureConstraint.Terminating<S, T>>
fun <S, T : Any> SenderScope<S>.optionally(default: T, parameter: StructureElement<S, Parameter<S, T>>): StructureElement<S, SignatureConstraint.Terminating<S, T>>