The growing efficiency and lower prices of computers make it possible to apply them more widely in the economy. However, the wide use of computers in every­ day life is hindered by a number of factors which constitute
what we shall call the "problem of contact" or of "talking". The difficulty is that languages used by computers differ substantially from users' languages and are not understood by specialists who are unfamiliar with
programming. This is why those specialists who use computers need the help of programmers to communicate. Since this form of communication has many more or less obvious shortcomings, great efforts have been made to find a
solution to the problem of contact. Two ap­ proaches can be distinguished here: (1) making the computer language similar to the natural language; (2) making the user's language resemble that of computers through formalizing
the former. This book deals with the first approach. We shall consider those systems which make it possible to "talk" with the user in limited natural language (LNL). The term "natural language" (NL) has been used in the title
of this book instead of LNL. The reason for not using the term "limited natural language" is that this term has two meanings: (1) a dialect of a natural language; (2) a formal language whose operators are expressed by words
taken from a natural language (e. g.