Trubar
 Model

1. Record
   consists of a unit description together with additional system information.
   It is a list of (key, value) pairs.

2. Key
   is a name of a data element in a record. There are two types of keys:
   • ordinary key - name of a property of unit considered in its description
   • system key - name of a system property of a record used by Trubar
   In a record there can be several pairs with the same key.

3. Catalog
   is a collection of records.

4. Group
   is a set of interrelated catalogs.