Maslov (1973); Oshiba (1972) proved that the cyclic closure of a context-free language is context-free.

Since context-free languages are ET0L, it follows that if L is context-free, then [C.

In other words, the general predicate can be taken to be 'is a property which it does not possess', and this contains a pronoun, which is a contextual item with no direct representation in a

context-free language.

This is a prototypical case of context-sensitive but not

context-free language.

Note that L(matched) is the

context-free language that consists of strings of matched parentheses and square brackets, with zero or more e's interspersed.

Unexpectedly, this question turns out to be intimately related to the complexity of deterministic

context-free language (DCFL) recognition.

It was attractive, says Pullum, because "a

context-free language can always be handled fairly efficiently.

Most of the

context-free languages differ in the amount of the resource (in this case, nondeterminism) that they require [6, 7].

After a quick overview of necessary prerequisites such as set and graph theory and the study of relations and general logic, the author follows a path that narrows its themes from chapter to chapter: from regular to

context-free languages and their models, to Turing machines and specific computation issues, including computability and decidability and general and context-sensitive grammars.

The families of recursively enumerable and

context-free languages are denoted by RE and CF respectively.

Topics covered in this volume include basic theory of computation, finite automata, properties of regular languages,

context-free languages, Turing machines, limits of algorithmic computation and computational complexity.

Their asymptotics is crucial for establishing (inherent) ambiguity of

context-free languages [Fla87], for the analysis of lattice paths [BF02], walks with an infinite set of jumps [Ban02] (which are thus not coded by a grammar on a finite alphabet), or planar maps [BFSS01].