[igraph] Reading a graph from file and counting graph edges

[R N]

Hi all,

This is a very simple question, but somehow I couldn't find any mention of it:
when reading a graph from file (e.g., GraphML), for connected vertices u and v,
should there be entries in the file for both edges (u,v) and (v,u), or
is it sufficient
to include only one of them (say, (u,v)), and the other one (v,u) is
automagically
assumed? I mean weighted undirected graphs.