Extra relation in text

[riannella]

Remove the extra "relation" from:

Explicit sub-properties of the abstract relation, relation and failure properties must be used, the choice depending on the subclass of Rule in question.

[simonstey]

Remove Replace the extra "relation" with "function"

[riannella]

This is a clear typo in the text and should be accepted as an Errata

[riannella]

Summary: In Section "2.6 Role Class", 4th para, the second "relation" should be replaced with "function"