An event queue is established for each list structure user that specifies an interest in sublist monitoring when the list structure is allocated. While sublist monitoring is in effect, the system will queue or withdraw event monitor controls objects (EMCs) to or from your event queue to indicate the empty or nonempty state of the sublists you are monitoring. Using event queue monitoring in conjunction with sublist monitoring allows you to determine whether the set of sublists that you are monitoring is empty or nonempty, and if one or more sublists is nonempty, to determine efficiently which sublist(s) those are.