Negation as failure is not currently possible with user-defined rules. The recommended way is with user-defined inferencing as described in the post you referenced: https://community.oracle.com/message/11167821#11167821.
Thank you for the quick response.
Is it possible to reuse the Triples inferred by user-defined inferencing in another rulebase and are their validity checked against the ontology ?
Yes. User-defined inferencing, a general-purpose extension to the native inference engine in Oracle Database, can be combined with another rulebase. And yes, one can validate (by running queries for example) user-defined inferencing results against an ontology.
Hope it helps,