I'm reading a piece on tracking algorithms and particle filters in particular, and stumbled upon the equation: $q(x_k(x_{k-1}^i,z_k)$. I think I understand most of the equation, but I can't understand what the second $($ is doing in the equation (the one between $x_k$ and $x_{k-1}$).
For reference, A Tutorial on Particle Filters for Online Nonlinear/Non-Gaussian Bayesian Tracking M. Sanjeev Arulampalam, Simon Maskell, Neil Gordon, and Tim Clapp, page 179, equation 52.