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Yet another solution is to define the planes as (non-compact) polyhedra:

sage: P_prime = Polyhedron(eqns=[(1, -1,2,1)])
sage: P_prime.Hrepresentation() 
(An equation (1, -2, -1) x - 1 == 0,)
sage: P_prime.Vrepresentation()
(A line in the direction (0, 1, -2),
 A line in the direction (1, 0, 1),
 A vertex at (0, 0, -1))
sage: P_prime.plot()