Null graph
[ 0-by-0 ]

Singleton graph
[        0 ]

Undirected singleton with loop
[        2 ]

Directed singleton with loop
[        2 ]

Small undirected graph
[        6        4        1
         4        0        1
         1        1        2 ]

Small directed graph
From and to types are the same:
[        1        1        0
         2        3        2
         1        0        0 ]
From and to types are different:
[        0        2
         3        4
         0        1 ]
