I12:A→ab 1 s→S b 12:S→C·bBA I:A→a·b 1:S→:S s→·CbBA a 13:S→CbB·A B A→·Aab I,:S→Cb·BA A→·ab I3:C→a B→:C A B→·Db C a C I1s;S→CbBA D→ A→A·ab I1:A→Aa·b lk:B→C Ir;B→D·b b 14: A→Aab l2:B→Db8