I am quite puzzled by the expression given in equation 21 (page 2110) in this paper, https://arxiv.org/pdf/1802.09188.pdf
Its LHS seems to be a measure $\nu_n^N$ and hence I guess it takes as argument measurable sets. Equation 21 is defining the measure $\nu_n^N$.
But its RHS is weighted sum of products of a measure $\mu_0$ and Markov kernels $R_{\gamma_i}$ As defined in its own equation 12, $R_\gamma$s are standard Markov kernels which needs 2 arguments a point and a measurable set.
So how is one to read this equation?
Is there a context to these kinds of expressions, that I am missing? Like, is this a familiar construction in some scenario? It would be great to get some pedagogic reference as to from where has this come!