Delimited control operators
===========================
+Here again is the CPS for `callcc`:
+
+ [callcc (\k. body)] = \outk. (\k. [body] outk) (\v localk. outk v)
+
`callcc` is what's known as an *undelimited control operator*. That is, the continuations `outk` that get bound to our `k`s behave as though they include all the code from the `call/cc ...` out to *and including* the end of the program.
Often times it's more useful to use a different pattern, where we instead capture only the code from the invocation of our control operator out to a certain boundary, not including the end of the program. These are called *delimited control operators*. A variety of the latter have been formulated.