In phase space, this is evident in the pure state Wigner quasiprobability distribution of this wavetrain, whose shape in x and p is invariant as time progresses, but whose features accelerate to the right, in accelerating parabolas B (x − B t) + (p / B − tB) = 0, W ( x , p ; t ) = W ( x − − B 3 t 2 , p − − B 3 t ; 0 ) = 1 2 1 / 3 π π B A i ( 2 2 / 3 ( B x + p 2 B 2 − − 2 B p t ) ) .