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TTYSCAN troubles.

    Date: 25 APR 1980 2257-EST
    From: ALAN at MIT-MC (Alan Bawden)

    GSB's solution is the one used by the LispMachine reader (including a
    catch/throw scheme to prevent doing the read twice).  No one has ever
    complained about the duplication of side effect issue.  I my opinion,
    if you are doing some kind of non-repeatable side effect at readtime,
    then you are probably doing something wrong.  This solution to rubout
    processing is the cleanest imaginable and it NEVER fails!  (Users can
    write the hairyest reader macros imaginable and it works like a
    charm.)  It isn't even all that expensive since it only happens during
    interactive READing and never when reading from a file.

    It sure is a shame that it is too late to do it the right way in MacLisp.

The other advantage with this approach is that it works for any arbitrary
"read" function, not just READ.