he <a href="BidirectionalGraph.html#sec:in-edges"><tt>in_edges()</tt></a>
function of the <a href="BidirectionalGraph.html">BidirectionalGraph</a>
interface provides access to all the in-edges of a vertex through <i>in-edge
iterators</i>. The <tt>in_edges()</tt> function is only available for the <tt>adjacency_list</tt>
if <tt>bidirectionalS</tt> is supplied for the <tt>Directed</tt> template
parameter. There is an extra cost in space when <tt>bidirectionalS</tt> is
specified instead of <tt>directedS</tt>.
