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External schemas in a schema-evolution environment for OODBs

External schemas are derived from the database conceptual schema; they can be used to simulate changes in it. Sometimes the final users' information requirements change: they need new information which cannot be derived from the information in the database. The solution put forward here is the definition of partially derived classes (capacity augmenting classes) which may contain non-derived information in the intension and moreover, unlike other systems, in the extension. When an external schema with partially derived classes is to be defined, the conceptual schema has to be modified in order to include the non-derived information. In order to avoid unnecessary modifications of the conceptual schema the use of a test environment for the definition of temporal external schemas is also proposed.