-xy(1-xy)-x^3y^3
=-xy(1-xy+x^2+y^2)
修改：
1-xy(1-xy)-x^3y^3
=1-xy+x^2y^2-x^3y^3
=1-xy+x^2y^2(1-xy)
=(1-xy)(x^2+y^2+1)