internal static IEnumerable <ParsedStatenode> GetDescendants(
this ParsedStatenode stateNode)
=> stateNode.CataFold(
atomic => atomic.Yield() as IEnumerable <ParsedStatenode>,
final => final.Yield() as IEnumerable <ParsedStatenode>,
(compound, subStates) =>
compound.Append(subStates.SelectMany(a => a)),
(orthogonal, subStates) =>
orthogonal.Append(subStates.SelectMany(a => a))).Except(stateNode.Yield());
