nd return its result.  Do not allow the invocation of
     continuations that would leave or enter the dynamic extent of the
     call to `with-continuation-barrier'.  Such an attempt causes an
     error to be signaled.

     Throws (such as errors) that are not caught from within PROC are
     caught by `with-continuation-barrier'.  In that case, a short
     message is printed to the current error port and `#f' is returned.

     Thus, `with-continuation-barrier' returns exactly once.

