The spec implies, though does not say explicitly, that call/cc can be implemented as follows:
(define (call/cc f) (call-with-non-composable-continuation f (default-continuation-prompt-tag))) However, the presence of any use of *call-with-continuation-prompt* with the default tag interferes with the usual *expected* operation of call/cc. So to keep people from shooting themselves in the foot and preserve the classic behavior of call/cc, my suggestion is to specify that call/cc use a private tag inaccessible to the user, and that in addition to the default tag, a prompt with the call/cc-private-tag be established at the start of every thread.
