graph with two unconnected vertices: 0
graph with two connected vertices, IGRAPH_VCONN_NEI_NEGATIVE: -1
graph with two connected vertices, IGRAPH_VCONN_NEI_NUMBER_OF_NODES: 2
graph with two connected vertices, IGRAPH_VCONN_NEI_IGNORE: 0
directed graph with two connected vertices, IGRAPH_VCONN_NEI_IGNORE: 0
line graph with 6 vertices: 1
full graph with 6 vertices IGRAPH_VCONN_NEI_IGNORE: 4
full graph with 6 vertices IGRAPH_VCONN_NEI_IGNORE, directed: 4
line graph with 3 vertices, 6 edges: 1
check error on graph with two connected vertices.
check error on graph with no vertices.
check error on graph with one vertex.
