Eilers (MPIA), Rix (MPIA), and I have spent two weeks now discussing how to model the kinematics in the Milky Way disk, if we want to build a forward model instead of just measuring velocity moments (Jeans style). And we have the additional constraint that we don't know the selection function of the APOGEE–Gaia–WISE cross-match that we are using, so we need to be building a conditional likelihood, velocity conditioned on position (yes, this is permitted; indeed all likelihoods are conditioned on a lot of different things, usually implicitly!).
At Eilers's insistence, we down-selected to one choice of approach today. Then we converted the (zeroth-order, symmetric) equations in this paper on the disk into a conditional probability for velocity given position. When we use the epicyclic approximations (in that paper) the resulting model is Gaussian in velocity space. That's nice; we completed a square, Eilers coded it up, and it just worked. We have inferences about the dynamics of the (azimuthally averaged) disk, in the space of one work day!