Incidence matrix with no rows and no columns:
directed: true
vcount: 0
edges: {
}
types: 

Incidence matrix for two vertices:
directed: true
vcount: 2
edges: {
0 1
1 0
}
types: 0 1

Incidence matrix for two vertices, multiple = true:
directed: true
vcount: 2
edges: {
0 1
0 1
0 1
0 1
0 1
1 0
1 0
1 0
1 0
1 0
}
types: 0 1

Incidence matrix for five vertices:
directed: true
vcount: 5
edges: {
0 3
0 4
0 4
1 2
1 2
1 2
1 3
1 3
1 3
1 3
1 4
1 4
1 4
1 4
1 4
2 1
2 1
2 1
3 0
3 1
3 1
3 1
3 1
4 0
4 0
4 1
4 1
4 1
4 1
4 1
}
types: 0 0 1 1 1

Same graph, IGRAPH_OUT:
directed: true
vcount: 5
edges: {
0 3
0 4
0 4
1 2
1 2
1 2
1 3
1 3
1 3
1 3
1 4
1 4
1 4
1 4
1 4
}
types: 0 0 1 1 1

Same graph, undirected:
directed: false
vcount: 5
edges: {
0 3
0 4
0 4
1 2
1 2
1 2
1 3
1 3
1 3
1 3
1 4
1 4
1 4
1 4
1 4
}
types: 0 0 1 1 1

Check error for negative element.
