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Added here is the NOTTHIS qualifier which, when it appears before a term in the body of a rule, means "don't use the current rule when attempting to satisfy this term as a subgoal". It's not clever, and it doesn't extend very far down the evaluation stack. In fact, in its current incarnation, the example here (where the rule head is defined in terms of itself immediately) is the only style that this will work with.
However, it does appear that this extra-logical markup is consistent, in as much as nothing which was provable before becomes unprovable now (if carefully applied) and nothing which is not derivable from known facts becomes provable.