sos_Aggregate is a superclass of all aggregation classes
(Array, Bag, Set, Mapping, and List).
It defines methods that are useful for all kinds of aggregations,
like for example a method that returns the cardinality of an
aggregation.
Besides, it is the basis for the cursor concept in SOS that enables
a user to scan every instance of an aggregation.
We distinguish between two kinds of aggregations:
Collections and Associations.
Instances of subclasses of class Collection consist of a
number of single elements, whereas instances of subclasses of class
Association consist of a number of pairs.
Therefore, Mappings are a specific form of associations,
and the other predefined aggregate types are specializations of the
class Association.