Node in the type inheritance hierarchy. Do not use this class for other purposes as it's definition is subject to change.
|event_tree<-id
associated with this node.
|event_node]*parent. By creating a new event_node the set
of valid names for the type event_id is extended and event
objects of this new type can be created.
The system defines a node named user for user-defined
event-types.
See also @event_tree, event->initialise
and event->is_a.