Objects based on this abstract class are used by the 'gk*' variants of the
the CPS manpage functions, to control their behavior. These objects are
expected to provide a method, 'again', which the functions will use to
re-invoke iterations of loops, and so on. By providing a different
implementation of this method, governor objects can provide such behaviours
as rate-limiting, asynchronisation or parallelism, and integration with
event-based IO frameworks.