Most computer users are familiar with the problems of sharing software with others, and the transfer of programs from one computing environment to another. Software represents an ever-increasing proportion of the cost of computing and these costs tend to nullify all the economic advantages flowing from the wider availability of cheap hardware. Years ago it was hoped that the widespread use of high-level programming languages would help in alleviating the problems of software production, by increasing productivity and by making it simpler for users with similar problems to be able to use the...