Simple implementation of nested groups

 Set::NestedGroups gives an implementation of nested groups,
 access control lists (ACLs) would be one example of
 nested groups.
 .
 For example, if Joe is a Manager, and Managers have access to payroll,
 you can create an ACL which implements these rules, then ask the ACL
 if Joe has access to payroll.
 .
 Another example, you may wish to track which city, state and country
 people are in, by adding people to cities, cities to states, and states
 to countries.
 .
 This module includes some facilities to integrate its data structures
 with either files in the local filesystem and with tables in a RDBMS.