see:
http://cgit.freedesktop.org/xorg/lib/libXi/tree/specs/encoding.xml#n1229
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XIproto.h#n676
(file-positions may become wrong if the spec or header change)
<field type="CARD8" name="modifier_device" />
<field type="CARD8" name="button" altenum="Grab" />
<field type="CARD8" name="grabbed_device" />
<field type="CARD8" name="modifier_device" />
<field type="CARD8" name="button" altenum="Grab" />
<field type="CARD8" name="grabbed_device" />
</request>
<!-- AllowDeviceEvents -->
</request>
<!-- AllowDeviceEvents -->