Rules applying to building a schema with HDSL

7 RULES APPLYING TO BUILDING A SCHEMA WITH HDSL 

We will now define certain rules for building the reference schema from the specification elements defined above. 

In a legal data structure no entity or property other than an entity of type WORLD (see below) may exist without being defined in the scope of another entity (no entity contains a LIST OF WORLD). 

Entities may refer only to entities of their environment. If entity E1 is in the scope of E2, and E2 is in the scope of E3 and so on until WORLD, then all entities in the scope of E2 and E3 and so on until WORLD constitute the environment of E1. 

There are three basic forms of expressing a relationship between entities of type E1 and type E2. 

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