At present the most commonly employed declarative representations of grammatical structure are context-free grammars (CFGs) as defined by Noam Chomsky (1956, 1957), because of their simplicity and efficient parsability.  Chomsky had argued that only deep linguistic representations are context-free, while surface form is generated by transformations (for example, in English passivization and in question formation) that result in a non-context-free language. However, it was later shown that on the one hand, unrestricted Chomskian transformational grammars allowed for computationally intractable and even undecidable languages, and on the other, that the phenomena regarded by Chomsky as calling for a transformational analysis could be handled within a context-free framework by use of suitable features in the specification of syntactic categories. Notably, unbounded movement, such as the apparent movement of the final verb object to the front of the sentence in “Which car did Jack urge you to buy?”, was shown to be analyzable in terms of a gap (or slash) feature of type /NP[wh] that is carried by each of the two embedded VPs, providing a pathway for matching the category of the fronted object to the category of the vacated object position. Within non-transformational grammar frameworks, one therefore speaks of unbounded (or long-distance) dependencies instead of unbounded movement. At the same time it should be noted that at least some natural languages have been shown to be mildly context-sensitive (e.g., Dutch and Swiss German exhibit cross-serial dependencies where a series of nominals “NP1 NP2 NP3 …” need to be  matched, in the same order, with a subsequent series of verbs, “V1 V2 V3 …”).  Grammatical frameworks that seem to allow for approximately the right degree of mild context sensitivity include Head Grammar, Tree-Adjoining Grammar (TAG), Combinatory Categorial Grammar (CCG), and Linear Indexed Grammar (LIG). Head grammars allow insertion of a complement between the head of a phrase (e.g., the initial verb of a VP, the final noun of a NP, or the VP of a sentence) and an already present complement; they were a historical predecessor of Head-Driven Phrase Structure Grammar (HPSG), a type of unification grammar (see below) that has received much attention in computational linguistics. However, unrestricted HPSG can generate the recursively enumerable (in general only semi-decidable) languages.