$rc: RecentChange being tagged when the tagging accompanies the action or null
$user: User who performed the tagging when the tagging is subsequent to the action or null
+'ChangeUserGroups': Called before user groups are changed.
+$performer: The User who will perform the change
+$user: The User whose groups will be changed
+&$add: The groups that will be added
+&$remove: The groups that will be removed
+
'Collation::factory': Called if $wgCategoryCollation is an unknown collation.
$collationName: Name of the collation in question
&$collationObject: Null. Replace with a subclass of the Collation class that