Author: Zachary Durber
MDL-23273 mod_choice: Using locks to ensure choice limits
http://git.moodle.org/gw?p=moodle.git;a=commit;h=e776b415f407f421a50414de1b617def2bf61e08
http://github.com/moodle/moodle/commit/e776b415f407f421a50414de1b617def2bf61e08
+ 2.9 en [cannotsubmit,mod_choice]