No vertices.
directed: false
vcount: 0
edges: {
}

One vertex, directed, with loops.
directed: true
vcount: 1
edges: {
0 0
}

Six vertices, directed, only edges from block 0 to 1 and 2 to 2.
directed: true
vcount: 6
edges: {
0 2
0 3
1 2
1 3
4 4
4 5
5 4
5 5
}

Six vertices, directed, only edges from block 0 to 1 and 2 to 2, no loops.
directed: true
vcount: 6
edges: {
0 2
0 3
1 2
1 3
4 5
5 4
}

Six vertices, undirected, only edges between block 0 and 1, and inside block 2.
directed: false
vcount: 6
edges: {
0 2
0 3
1 2
1 3
4 4
4 5
5 5
}

Six vertices, undirected, only edges between block 0 and 1, and inside block 2, no loops.
directed: false
vcount: 6
edges: {
0 2
0 3
1 2
1 3
4 5
}

Check for nonsquare matrix error handling.
Check for preference matrix probability out of range error handling.
Check for nonsymmetric preference matrix for undirected graph error handling.
Check for incorrect block size vector error handling.
Check for negative block size error handling.
Check for sum of block sizes not equal to number of vertices error handling.