I am reading this paper on the Hierarchical Clustering.
The author introduces cluster intra-compactness indicator and it is formulated as follows
$$RR_{intra}(S) = \sum_{i \in D \\ C_{k} \in S | i \in C_{k}}p_{i}' * RR_{i, C_{k}\setminus i},$$ where $D$ is the set of diagnoses, $i$ is the index of some diagnosis, $p_i$ is the empirical probability of a diagnosis occurrence. $S = \{C_1, \ldots,C_N\}$ is the clustering solution of a set of $N$ clusters denoted $C_k$, $k \in\{1, \ldots, N\}$, $RR$ is the relative risk.
I understand the formula within, but I do not understand how to interpret two summation indices stuck under each other? Any hints?