+// The po_iterator_storage template provides access to the set of already
+// visited nodes during the po_iterator's depth-first traversal.
+//
+// The default implementation simply contains a set of visited nodes, while
+// the Extended=true version uses a reference to an external set.
+//
+// It is possible to prune the depth-first traversal in several ways:
+//
+// - When providing an external set that already contains some graph nodes,
+// those nodes won't be visited again. This is useful for restarting a
+// post-order traversal on a graph with nodes that aren't dominated by a
+// single node.
+//
+// - By providing a custom SetType class, unwanted graph nodes can be excluded
+// by having the insert() function return false. This could for example
+// confine a CFG traversal to blocks in a specific loop.
+//
+// - Finally, by specializing the po_iterator_storage template itself, graph
+// edges can be pruned by returning false in the insertEdge() function. This
+// could be used to remove loop back-edges from the CFG seen by po_iterator.
+//
+// A specialized po_iterator_storage class can observe both the pre-order and
+// the post-order. The insertEdge() function is called in a pre-order, while
+// the finishPostorder() function is called just before the po_iterator moves
+// on to the next node.
+
+/// Default po_iterator_storage implementation with an internal set object.
+template<class SetType, bool External>