To build libraries that use send/suspend, the library code must reside inside
a unit that imports send/suspend. Servlets that use the library must
link the main servlet with the code from the library using a compound-unit.
helper.ss
This file provides the function _get-number_ that uses send/suspend
to interact with the consumer.
helper-sig.ss
This file provides a unit signature required for linking to the helper
library.
add.ss
The add servlet links to the helper unit and calls get-number.
multiply.ss
The multiply servlet also uses the helper library.
