WITH-STANDARD-IO-SYNTAX binds *READTABLE* to SB-IMPL::*STANDARD-READTABLE*,
which may then possibly get modified.
Even though it is verboten by the spec to modify the standard-readtable,
I think it'd best if SBCL tried to save a user from shooting in his foot
by signalling an error in such a case.
As I'm looking at the readtable related code for other reasons anyway,
I'd like to implement such explicit checking.
a) Should SET-MACRO-CHARACTER etc. check for its argument being EQ to
b) Should I introduce a new slot READONLYP which can be set by the
user as an extension, and which is set for *STANDARD-READTABLE* by
default, and which results in an error when such a readtable is
tried to be modified?