Warning of upcoming incompatible user-interface change.

[Daniel L. Weinreb <dlw at MIT-AI>]

In System 56 (which is not released yet) and after, the character Control-Z will
no longer be intercepted specially, the way it is now.  Instead, the ABORT
character will do what Control-Z used to do.  Right now, in system 55, both
Control-Z and ABORT work; Control-Z won't be supported in system 56.  This
is because Control-Z is not mnemonic at all except to users of a certain
operating system used only at MIT that most people have never heard of.  In
general, the rule for the Lisp Machine user interface will be that ABORT is
used for aborting, quitting, flushing, and things like that.

By the way, what ABORT does now, and will continue to do, is to throw
to the tag SI:TOP-LEVEL.  This is done in a low-level function of keyboard
typein.

The CALL key is completely unrelated to Control-Z, and is not affected
by this change.

An analogous change was made in system 55 (the current system) to the BREAK
function; you can now use ABORT and RESUME to quit out of or return from
a breakpoint.  The magic "altmode-P" symbol is still supported but obsolete.