Publishing

How a handler answers a message and publishes to other destinations.

A handler publishes in two ways. Returning a value is a reply, the default when the handler answers on one destination. Taking a publisher as an Out slot is for a destination chosen at run time, or several at once, another broker's included. Either way the handler never holds a publisher that is not connected: the mount site names a publish policy, and startup pairs it with the connected broker. A message type declares its own destination, and one builder publishes every message.