Constraints and limitations
any string has a node of its own
- word-form, punctuation mark, etc.
- AuxV, AuxP, AuxC, AuxX, AuxG…
reflecting the coordination and apposition relations
- so called third dimension of the graph in the plain tree (X_Co, X_Ap, X_Pa, where X is one of analytic functions, such as Sb, Obj, Adv, etc.)