ld be free use any ordering for the pair of vertices
in an out-edge. For example, if you call <tt>out_edges(u, g)</tt>, and
<tt>v</tt> is one of the vertices adjacent to <tt>u</tt>, then the
implementation would be free to return <tt>(v,u)</tt> as an out-edge
which would be non-intuitive and cause trouble for algorithms.
Therefore, the extra requirement is added that the out-edge connecting
<tt>u</tt> and <tt>v</tt> must be given as <tt>(u,v)</tt> and not
<tt>(v,u)</tt>.

<H3>Concept Checking Class</H3>

<P