Minor issue: What is a "substitute" and how do you "designate" it? Is the substitute what
delegate-sheet-delegate returns? Does't this want an initarg for the substitute? | SWM

=> delegate-sheet-delegate sheet [Generic Function]
=> (setf delegate-sheet-delegate) delegate sheet [Generic Function]
This may be set to another recipient of events dispatched to a sheet of class delegate-sheet-
input-mixin. If the delegate is nil, events are discarded.

8.2 Standard Device Events

An event is a CLIM object that represents some sort of user gesture (such as moving the pointer
or pressing a key on the keyboard) or that corresponds to some sort of notification from the
display server. Event objects store such things as the sheet associated with the event, the x and
y bposition of the pointer within that sheet, the key name or character corresponding to a key
on the keyboard, and so forth.

Figure 8.1 shows all the event classes.

=> event [Protocol Class]
The protocol class that corresponds to any sort of "event". If you want to create a new class
that behaves like an event, it should be a subclass of event. Subclasses of event must obey the
event protocol.

All of the event classes are immutable.

=> eventp object [Function]
Returns true if object is an event, otherwise returns false.