(69) S → NP⁀VP; VP → V⁀NP; (70) S → {NP, VP}; VP → {V, NP}. In (70), no order is assigned to the elements on the right-hand side of the rule; thus, {NP, VP} = {VP, NP}, altough NP⁀VP ≠ VP⁀NP [...] The rules (69) convey more information than the corresponding rules (70), since they not only define an abstract system of grammatical relations but also assign an abstract underlying order to the elements [...] Proponents of 'set systems' such as (70) have argued that such systems are more "abstract" than 'concatenation systems' such as (69), and can lead to a study of grammatical relations that is independent of order, this being a phenomenon that belongs only to surface structure.
- Chomsky (1969), a pag.124