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I would suggest something like this

k=2
Q=QuadraticForm(ZZ,2,[1,-1,1]);Q
m=Q.short_vector_list_up_to_length(k^2+1)
m[k^2]

which is $A_2$ for some $k$ which gives the result in the basis $e_1=(1,-1,0), e_2=(0,1,-1)$.