Directed null graph
directed: true
vcount: 0
edges: {
}

Undirected null graph
directed: false
vcount: 0
edges: {
}

Directed singleton graph
directed: true
vcount: 1
edges: {
}

Undirected singleton graph
directed: false
vcount: 1
edges: {
}

Edgeless graph
directed: true
vcount: 3
edges: {
}

Small DAG
directed: true
vcount: 9
edges: {
3 1
3 2
4 1
5 0
6 0
6 1
6 2
6 3
7 0
7 1
7 2
7 3
7 6
8 0
8 1
8 2
8 3
8 6
8 7
}

Small directed multigraph with cycles 
directed: true
vcount: 10
edges: {
0 1
0 2
0 3
0 4
0 5
0 6
1 0
1 2
1 3
1 4
1 5
1 6
2 0
2 1
2 3
2 4
2 5
2 6
3 4
4 3
5 6
8 7
}

Small undirected graph
directed: false
vcount: 6
edges: {
0 1
0 2
1 2
3 4
}
