Set v of ZG-generators for the submodule '

(-1+x)e_a

(-1+y)e_b

(-1+xy)e_c

(-x+y)e_d

(1+xy)e_a +(-1-x²)e_c +(1+x²)e_d

(-1-yx)e_b +(y+xyx)e_c +(1+yx)e_d

The main table

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