20.若集合A1A_{1}A1,A2A_{2}A2满足A1⋃A2=AA_{1}\bigcup A_{2}=AA1⋃A2=A,则称(A1(A_{1}(A1,A2)A_{2})A2)为集合AAA的一种分拆,并规定:当且仅当A1=A2A_{1}=A_{2}A1=A2时,(A1(A_{1}(A1,A2)A_{2})A2)与(A2(A_{2}(A2,A1)A_{1})A1)为集合AAA的同一种分拆,写出集合A={aA=\{aA={a,b}b\}b}的不同分拆.