https://www.youtube.com/watch?v=Ju3KKu_mthg

This video, which was shared on the Scheme discord channel, explains how
call/cc captures the state of the stack.

I think that, in the language of the video, I changed the SRFI library
code to ensure that, if the stack/continuation is captured, the library
code doesn't change the state of the data on the captured stack (using
set!, vector-set!, etc.).

I figured that, instead of trying to say what "call/cc safe" means in
general, I'd say how I changed the library code.

Brad