sexderi - Definition of subsets

Any number of extents can be defined as subset of an extent. Subsets must be defined as extents based on the same type as the current extent or on a specialized type of type of the current extent, when the extent is weak-typed.

Subset/superset relations are bi-directional and need to be defined only in one direction. As long as the set relations are consistent, one may define also both directions.

Extended set relations as union and distinct can be defined by means of additional options.