🚀 go-pugleaf

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.