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New constants needed

I have no particular objection to your proposed new constants, but I
don't see the reason for String-Char-Code-Limit, as long as each
implementation is required to provide a correct implementation of
String-Char-P.  Requiring such a limit would implicitly require that the
string chars occupy a continuous range starting at 0 and with no holes.
I don't think we require that now, and it might be awkward for some odd
encodings -- I'm not sure what EBCDIC does.  If it were up to me, we'd
just define the language in terms of ASCII and be done with it, but if
we are going to straddle the fence on this (as Guy has done so
carefully), we must be careful not to fall off.

I suppose that if we were to define these limits as bounds on the set of
legal string or readtable characters, without saying that characters
within these bounds are necessarily legal, then we can at least use them
to see how large certain tables and fields have to be.  CHAR-CODE-LIMIT
already gives us a loose bound of this sort, but perhaps these other
numbers would give a somewhat tighter bound for some implementations.

-- Scott