For undirected graphs, head and tail is not defined. In this case
tail_of returns vertices incident to the supplied edges, and
head_of returns the other end(s) of the edge(s).
tail_of(graph, es)The input graph.
The edges to query.
A vertex sequence with the tail(s) of the edge(s).
Other structural queries: [.igraph,
[[.igraph, adjacent_vertices,
are_adjacent, ends,
get.edge.ids, gorder,
gsize, head_of,
incident_edges, incident,
is_directed, neighbors