Figure 3.

Sub-network: $X_{in,a}$, $X_{out,a}$, $X_{in,b}$, $X_{out,b}$, $X_{in,c}$, and $X_{out,c}$ are all random variables; $N_0$ is the number of values taken by $X_{in,a}$ and $X_{in,b}$; $N_1$ is the number of values taken by $X_{out,a}$ and $X_{out,b}$; and $N_2$ is the number of values taken by $X_{out,c}$.