A behaviour module for implementing a server, which maintains a local tree (key-value map) as its internal state and reacts on changes on another part of the (global) tree. A processor is supposed to be

• mounted on a TreeServer at a path via the TreeServer.mount_source/1 function, normally during the c:init/1 callback.
• registered on one or more paths on a TreeServer via the TreeServer.register_sink/1 function, at any time.

It is then supposed to do three things:

• Reply to query requests by returning the local tree in a mounted state (done by the handle_query functions in this module)
• Notify the hosting TreeServer about updates in the local tree via the TreeServer.notify/2 function
• React to notifications (casts) received from the hosting TreeServer via the handle_notify functions