Entity Sets (Group) & Identifier Translation
The Group and IdTranslator classes perform entity subsetting and user identifier translation. Functions exist to convert Group objects to IdTranslator objects and vice versa so that the most useful form of an entity subset may be represented.
The Group object is useful for archiving entity sets if there are a small number of sets and the sets are therefore dense. Internally a Group object allocates a slot for each entity, such as the total number of nodes or elements, in the model. The total memory required for set storage will be proportional to the number of groups and this memory could be large for a large number of groups.
The IdTranslator object is useful for archiving entity sets if there are a large number of sets and the sets are therefore sparse. Internally an IdTranslator object allocates a slot for each entity in the set. If the sets are disjoint, the total memory required for set storage will be generally bounded by the total number of entities in the model.
