This page is part of the web mail archives of SRFI 13 from before July 7th, 2015. The new archives for SRFI 13 contain all messages, not just those from before July 7th, 2015.
The new caveat on substring, that it be allowed to share storage, seems strange to me, since it could noticeably and adversely affect pre-existing programs that rely on the old behavior. Plus, the SRFI specifies "/shared" variants of other calls to explicitly call out the possibility of sharing. So, why not leave substring alone, and add a substring/shared which is allowed (but not required) to share storage with its argument? -dan