[Xcb] How do I distinguish XKB events?

[me at beroal.in.ua]

Hello. I have a question regarding the xkb extension.

"The X Keyboard Extension: Protocol Specification" says in "Chapter 16. 
XKB Protocol Requests/Events":
 > XKB uses a single X event code for all events
 > and uses a common field to distinguish XKB event type.
I don't know what the heck is the "common field". I guess that in XCB, I 
need to conver "xcb_generic_event_t *" to something like 
"xcb_xkb_generic_event_t *" where I'll find the common field. But I 
can't find such a struct, though I see several structs named 
"xcb_xkb_$X_event_t". So how do I distinguish XKB events?