163303e7_54_3__Event_types__Description
[Type] 4 [Name] Button [Description] 
The user pressed one of the mouse buttons. The detail part specifies which button. For mouse wheel support under Linux, use Button-4 (scroll up) and Button-5 (scroll down). Under Linux, your handler for mouse wheel bindings will distinguish between scroll-up and scroll-down by examining the .num field of the Event instance; see Section 54.6, “Writing your handler: The Event class”.
54.3. Event types
Description