var("a")
A2 = matrix([[1,1,2,-1], [1,0,2,-a], [1,1-a,2,-3], [0,2,-1,a]])
b2 = vector([0, -1, -2, 2])

Ab = A2.augment(b2)
show(Ab)

M1 = copy(Ab)
M1.add_multiple_of_row(1,0,-1)
M1.add_multiple_of_row(2,0,-1)
show(M1)

M2 = copy(M1)
M2.add_multiple_of_row(2,1,-a)
M2.add_multiple_of_row(3,1,2)
show(M2)

M3 = copy(M2)
M3.swap_rows(3,2)
show(M3)