 be accessed with the <a href="EdgeListGraph.html#sec:edges"><tt>edges()</tt></a>
function of the <a href="EdgeListGraph.html">EdgeListGraph</a> interface.
Similar to the <tt>vertices()</tt> function, this returns a pair of iterators,
but in this case the iterators are <i>edge iterators</i>. Dereferencing an edge
iterator gives an edge object. The <tt>source()</tt> and <tt>target()</tt>
functions return the two vertices that are connected by the edge. Instead of
explicitly creating a <tt>std::pair</tt> for 