Derivation by restriction - can I change element reference?

[Grisha Golberg]

Hi all,

Here's in a nutshell the structure I'd like to define:

1. Connection, an abstract type has a URL
2. JDBC connection is a Connection with username and password
3. FileConnection is a Connection
4. A QueryNode is an abstract type, it has a reference to Connection
5. A JDBCQueryNode contains a reference to JDBCConnection
6. A FileQueryNode contains a reference to FileConnection
7. A Project contains several connections and QueryNodes.

So, I want to make sure that only JDBCQueryNode can reference only JDBCConnection.