Expands to code that sets 'callee' as the widget in 'place', saves
the continuation, and returns from the delimited computation. When
'callee' answers, restores the original widget in 'place' and
reactivates the computation. If 'wrapper-fn' is present, passes it the
new callee and sets the return value as the value of a place. By
default 'wrapper-fn' is simply an identity function.