referenceOrNull
fun <SourceT : ITypedNode, TargetT : ITypedNode> referenceOrNull(input: IMonoStep<SourceT>, link: ITypedReferenceLink<TargetT>): IMonoStep<TargetT?>
fun <SourceT : ITypedNode, TargetT : ITypedNode> referenceOrNull(input: IFluxStep<SourceT>, link: ITypedReferenceLink<TargetT>): IFluxStep<TargetT?>