Upcoming user-interface change

[Moon at MIT-AI (David A. Moon)]

In system 56, which is not quite ready for release to users yet, the use
of the Abort key has been made more consistent and useful.

When the Abort key is typed, the program which reads it aborts what it is
doing and throws out to "command level".  BREAK and the error handler
are command levels, and so is anything else that *CATCHes SI:TOP-LEVEL.

The effect is that if you type Abort repeatedly, each successive abort
will go back to an earlier break or error-handler, until you get back to
the top level (read-eval-print loop in a Lisp listener, the editor in an
editor).

This use of the Abort key is turned off by certain programs, such as the
editor.  However, it is always in effect when typing at read-eval-print
loops and the error handler.

The error-handler's meta-Z command has been flushed, since it never worked
very well and since abort now does what meta-Z was supposed to do.

Currently control-Z is the same as Abort, however this is scheduled to
be flushed.  Also the altmode-G feature of Break has been flushed (I
assume no one knows about this, I was under the impression it had been
flushed about 3 years ago).

Control-Abort resets the process, i.e. always throws all the way out of
everything, and then starts the process over again if it is the kind
(like a Lisp listener) that makes sense to start over.  Control-Abort
also happens immediately, whereas Abort happens when it is read.