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How literally do you mean that BREAK should enter the debugger?
On Maclisp and even now on the LispM, when you type Break, you are at a place
where you type s-expressions, not commands. On the LispM, you have a wide
enough character set to be able to merge the break facility with the debugger
and still win, but in ASCII land, this will be a lot tougher. I, for one,
will be unhappy with BREAK putting me somewhere where the first thing I have
to type is a command to get me to a vanilla read eval print loop style 
debugging environment. I suggest that the wording should be weakened so that
BREAK is constrained only to pause and allow you access to debugging state
of some sort, but it should be up to the implementation whether a debugger
or read eval print loop is entered. I think this is in the domain of error
handling and is something and should at this point be left to individual

Alternatively, I would not mind (I would even prefer) two similar commands,
like DBG and BREAK on the Lisp Machine, so I could call the one which was
appropriate for the kind of debugging I am doing. I will almost always know
in advance which of these debugging environments I want. If they happen to
be different entry points to the same program (as I suspect will eventually
be the case with the LispM), that's fine. It's just a question of intent.

The former suggestion is probably most practical at this point.

Other than that, I think error.proposal.9 looks fine.