A downward-only version of Scheme's call with current continuation.
function of one argument, the exit procedure.
callCC provides a non-local exit mechanism that can be useful
for early termination of a computation.
fun with one argument, an exit function. The exit
function takes a single argument, the intended return value. If the
fun calls the exit function then the call to
callCC immediately returns, with the value supplied to the exit
function as the value returned by
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