| SELECT_DESCENDANT of StateSet.t * Ptset.Int.t * Tree.unordered_set
| SELECT_FOLLOWING of StateSet.t * Ptset.Int.t * Tree.unordered_set
| TAGGED_CHILD of StateSet.t * Tag.t
| SELECT_DESCENDANT of StateSet.t * Ptset.Int.t * Tree.unordered_set
| SELECT_FOLLOWING of StateSet.t * Ptset.Int.t * Tree.unordered_set
| TAGGED_CHILD of StateSet.t * Tag.t