Set v of ZG-generators for the submodule '

(-1+x)e_a

(-1+y)e_b

(-x+y)e_d

(1+xy)e_a +(-1-x²)e_c +(1+x²)e_d

(-1-x)e_b +(1+y²+xy³)e_c +(x+xyx+y²x)e_d