Hmm, OK, thanks. I'm in a bit of a fix, though, since I want essentially :SNIPPET but narrowed to the defun -- Scheme48 provides the source code for every expression that will have a continuation, but in a form that doesn't necessarily correspond with the source text literally (since it's in S-expression form, not text form), so I want to use slime-isearch. This is more information, too, than :CALL-SITE can deal with. Would it make sense to have, or would anyone object to having, another hint, say :CALL-SNIPPET, which would work like :SNIPPET but narrow to the defun first? The implementation is trivial -- it would simply be the patch I sent, but without the removal of :SNIPPET.
(Of course, the right thing would be for Scheme48 to provide better proper source location in the form of paths down the S-expression tree. But that requires changes to Scheme48 itself, and my efforts to get changes on the Scheme48 end haven't been very fruitful, unfortunately...)