1.0.23.57: rewrite bug 217
* Case A is a non-issue. SBCL interprets "unpredictable but harmless"
to mean "anything, but heap will no be corrupted, data lost, etc"
-- that is, an error may or may not be signalled, but it the lisp
session will remain sane and operable.
* Case B is negated by the portability argument: since specifying both
is unspecified, we should not build useful-seeming extensions on top
of it, as it will only create portability problems for users.
* Case C remains partially valid, except for the runtime WARNINGs.
Integrated into the new text.