  The essential idea is that the propagator acts so that no matter how close together \(\bx(0)\) and \(\by(0)\) are the trajectory initiating from \(\by(0)\) will eventually diverge by \(\varepsilon\) from the trajectory initiating from \(\bx(0)\). However, WSD does not specify the rate of divergence (it is compatible with linear rates of divergence) nor does it specify how many points surrounding \(\bx(0)\) will give rise to diverging trajectories (it could be a set of any measure, e.g., zero). 