// Rollup handles regular scheduling and traffic-cop operations for
// the creation of nodes at regular intervals. Specifically, the creation
// of nodes that summarize other stuff going on across the site.
//
// One example might be a node posted once every week that lists the new
// users who've registered for the site. Another might be a daily node that
// lists all the posts a particular user has made on twitter.com. The module
// is set up to prevent 'empty' summaries from being posted.
//
// In addition, it provides a couple of key points of extensibility:
//
// * hook_rollup_intervals() allows modules to define custom time intervals
//   for rollups to execute. Defaults are each day, each week, and each month.
//   An example of a custom interval might be 'every weekday'.
//
// * hook_rollup_domains() allows modules to define custom "content domains."
//   Domains are things like 'the entire site' or 'per user' or 'per organic
//   group.'
//
// * hook_rollup_check() allows modules to check whether they have content to
//   add to a specific rollup node.
//
// * hook_rollup() allows modules to alter, or add related records, to a new
//   rollup node.
//
// * hook_rollup_definitions() allows modules to expose pre-defined scheduled
//   rollups. (For example, a pre-defined weekly summary of new weblinks could
//   be exposed by links.module).