Footnotes

 

(1)

German Research Center for Artificial Intelligence GmbH, Stuhlsatzenhausweg 3, D-66123 Saarbrücken, Germany, Tel. -49-681-302-5317  

(2)

Notice that it is crucial to consider all sentences instead of for instance all axioms. For instance, extending of a theory of natural numbers by two axioms c = 0 and c = s(0) would identify 0 and s(0) or render the theory inconsistent.