4. Immediate vertex selectors

4.1. igraph_vss_all — All vertices of a graph (immediate version).
4.2. igraph_vss_none — Empty vertex set (immediate version).
4.3. igraph_vss_1 — Vertex set with a single vertex (immediate version)
4.4. igraph_vss_vector — Vertex set based on a vector (immediate version).
4.5. igraph_vss_seq — An interval of vertices (immediate version).