Bug reports of a few days about completely weird things happening in ZTOP mode

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

In the version of ZWEI on system 31.1, with microcode 674, on LISP Machine One:

I understand now; there is an invisible BP in the buffer which is where it's
going to read from next.  If I freely edit in the buffer, for example if I
start typing in an S-expression then decide I want to do something else first
and so typed something else before it, I can very easily back up over that
invisible BP and type some of my stuff where it isn't going to be seen.
For instance if that BP is at the beginning of a line I can easily insert
things before that line without obviously going back into what has already
been typed out, by typing control-A control-O  (I've tested this).

Something needs to be done about this.

Also I don't think it's necessary any more to type everything out twice,
since the editor and PRINT agree in 31.1 about continuation lines.