Use this if you are using igraph from R
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)
graph | 
 The input graph.  | 
es | 
 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()