(4)消去单环 S21 E 1-ST:a 1-S2 b S /1-S「g (5)分裂a1 1 1-S2TL S - SiT. 1-S 1- S11Gg 1 a1 b2 g g L S S - G G G 11 12 1 S L S - 22G 21 E 1 （4）消去单环 1- S11Gg 1 a1 b2 g g L S S - G G G 11 12 1 S L S - 22G 21 E 1 L S S - 22G 21 1 1 a' （5）分裂a1