3.16.3 Event Forward Declaration

   (135) <event_forward_dcl> ::= [ “abstract? ] “eventtype? <identifier>

   A forward declaration declares the name of an event type without defining it. This permits the definition of event types that refer to each other. The syntax consists simply of the keyword eventtype followed by an <identifier> that names the event type.

   Multiple forward declarations of the same event type name are legal.

   It is illegal to inherit from a forward-declared event type whose definition has not yet been seen.