sum(0,0)=0, sum(s(0),s(0))=s(s(0)), sum(0,s(0))=s(0), sum(s(s(0)),0)=s(s(0)), sum(s(0),0)=s(0), sum(s(0),s(s(0)))=s(s(s(0))), sum(s(s(0)),s(s(0)))=s(s(s(s(0)))), sum(s(0),0)=s(0), sum(s(s(s(0))),s(0))=s(s(s(s(0)))), sum(s(s(s(0))),s(s(0)))=s(s(s(s(s(0)))))