The Concept of a Context-free Language was first introduced by Chomsky in 1959 [Ch 3] (Refers to N. Chomsky, 3d listing, in the References and Related Papers" at the end of the book. The same shorthand form used in the text, for example, [Ch 3], precedes the corresponding entry in the "References.") in an attempt to find a reasonable mathematical model of natural languages such as English, French, etc. In the period 1959-1960, several papers developing the theory were written [Ch 3, 4; CM; BGS; BPS]. In late 1960, it was discovered that the "ALGOL-like" languages, that is, the languages defined by Backus normal form (the metalanguage used to describe the widely publicized programming language ALGOL-60), were identical with the context-free languages. Since then, there has been a flurry of activity in the theoretical development of context-free languages..." [from the preface]