Hmm, I think that's basically a try/catch mechanism. That is, call_cc gives you a block which includes fred. Calling fred will pop out of the whole call_cc block, even if fred was called by a subroutine deeply nested in the calling sequence. That is, it generates a "throw" keyword for you that's specific to this "try" scope. The argument to fred is used to signal exception data back to the point after call_cc, which can use that to distinguish success from failure.
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