HelenOS sources

USB_HIDUT_PAGE_BUTTON  226 uspace/drv/hid/usbhid/mouse/mousedev.c 	    usb_hid_report_path_append_item(path, USB_HIDUT_PAGE_BUTTON, 0);
USB_HIDUT_PAGE_BUTTON  288 uspace/drv/hid/usbhid/mouse/mousedev.c 	usb_hid_report_path_append_item(path, USB_HIDUT_PAGE_BUTTON, 0);
HelenOS homepage, sources at GitHub