A module containing broadcast channels (one-to-many). Whereas a one-to-any
channel features one writer sending a single value to one (of many) readers, a
one-to-many channel features one writer sending the same value to many
readers. So a one-to-any channel involves claiming the channel-end to ensure
exclusivity, but a one-to-many channel involves enrolling on the channel-end
(subscribing) before it can engage in communication.
A communication on a one-to-many channel only takes place when the writer
and all readers currently enrolled agree to communicate. What happens when
the writer wants to communicate and no readers are enrolled is undefined
(the writer may block, or may communicate happily to no-one).