### Set

 context $n_X,n_Y\in\mathbb N$
 definiendum $B \in \mathrm{it}(n_X,n_Y)$
 postulate $B \in \mathrm{Matrix}(n_X,n_Y,\mathbb N)$

### Discussion

If the indices $i,j$ label two vertices belonging to the partitions $X,Y$ of a finite bipartite graph, respectively, then the value $B_{ij}$ determines the number of edges joining them.