# depth-first traversal of a graph

my ($visited_color);

traversal dfs($precode, $postcode) from Node to Node
do
initialize:
	$visited_color++;
Node.pre:
	return if $self->{color} == $visited_color;
	&$precode($self);
	$self->{color} = $visited_color;
Node.post:
	&$postcode($self);
endtraversal

my ($n1,$n2, $n3);

# construct a small example graph with a loop
$n1 = new Node (neighbors => [(new Node), (new Node)]);
$n2 = new Node (neighbors => [new Node (neighbors => [$n1])]);
push @{$n2->{neighbors}}, $n2;  # make a loop in the graph

# traverse it
$n2->dfs(sub { print "pre $_[0]\n" }, sub { print "post $_[0]\n" });