is fulfilled by <i>visitors</i>.
<p>A visitor is like a functor, but instead of having just one &quot;apply&quot;
method, it has several. Each of these methods get invoked at certain
well-defined points within the algorithm. The visitor methods are explained in
detail in Section <a href="visitor_concepts.html">Visitor Concepts</a>. The BGL
provides a number of visitors for some common tasks including a predecessor
recording visitor. The user is encouraged to write his or her own visitors as a
way of extendi