The social depressive has been around at least since psychoanalysis became trendy and Valium became essential for sophisticated housewives.
On the other hand, its understanding became essential for some engineering applications, mainly in metallurgy and latent thermal energy storage where phase change materials are used.
The possession of a streetcar line became essential for a growing town or small city, and the larger city streetcar systems extended their lines farther and farther out into the suburbs.
This invention led not only to more cleverly designed file systems but also to the development of database and information retrieval systems, which later became essential for storing, retrieving, and transmitting large amounts and wide varieties of data across the Internet.
The South Korean envoy, Chun Yung-woo, helped shaped the discussions about the amount of oil and also suggested "more oil for deeper denuclearization". This link became essential for the Americans, who were distrustful of any deal that provided North Korea with oil without linking it to disarmament.
Such giant machines became essential for the printing of paperbacks, but the problem remained of printing economically those "short runs" of 3,000 or so in which the works of new authors, from whom many of the important books of the future must come, are normally tried out.
