I spent much of the day working on an old-school perturbation-theory calculation. I consider a cold tidal stream in a host galaxy potential, perturbed by the close passage of a point mass. In the limit of small perturbations of very cold streams, this calculation has only two free parameters: the angle between the direction of the stream and the velocity of the perturber (in the comoving frame of the stream or equivalent), and the time since the impulse (in some scaled time units related to the mass and velocity of the perturber or amplitude of the perturbation). This is all very idealized, but actually, to zeroth order, I think I may have exhaustively described all possible perturbations to cold streams. Now the idea is to use this to constrain the substructure in our Galaxy with observations of cold streams.