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()