1.0.23.57: rewrite bug 217

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.