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Aucbvax.1989 fa.works utzoo!duke!decvax!ucbvax!dlw@MIT-AI Mon Jun 29 23:46:20 1981 Interupting a workstation session I agree that it is hard for a user to keep track of a stack of interruptions. Having to maintain a mental model of such a stack, and having to remember what "exit this command level" will do, is a real pain. Most interactive systems I have used have suffered from this problem. The Lisp Machine solves the problem by having all of the user's activities be at the same "level". There isn't any command processor that "calls" programs which then "return" to the command processor; you just move "sideways" from one thing to another. No stacks are involved. (Actually there are still a few stacks in the system, but they are being removed.) (There are some commands that mean "switch back to the previous thing I was doing", which you sometimes want, but nothing forces you to use these commands. (If ùou gives such a "previous thing" command over and over, it switches between the same two things, in case you were wondering.))

Not only is this easier to use, but it is more powerful. If you are reading your mail and you are interrupted by a phone call, you can go handle the phone call, and then put the caller on "hold" and go back to reading your mail, and then get back to the phone call. That is, you need not maintain a last-in first-our ordering among your actitivies. This is one of the things I think is most valuable about the Lisp Machine's overall user interface structure.

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