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### Set

 $V$ … countable set
 $\phi\in\mathrm{it}$
 $\mathrm{dom}\ \phi = V$
 $v,u\in V$
 $\phi(v)\subseteq V$ $u\in\phi(v)\implies v\in\phi(u)$

### Discussion

The value $\phi(v)$ denotes the set of vertices which are connected to $v$.

The adjacency lists describe simple graphs.