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