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In section 3.2, after equation (13) we stated 'In the special case where δ does not depend on time and α(0) = 0, the solution is α(t) from equation (6).' This is only correct if the phase ϕ in (13) is substituted by −ϕ + π. Since this phase can be defined arbitrarily, its change has no effect on the physics and all other results remain valid.