Graphlib.dfs_identity_visitor
base class with all methods defaults to nothing.
method start_tree : 'n -> 's -> 's
method enter_node : int -> 'n -> 's -> 's
method leave_node : int -> 'n -> 's -> 's
method enter_edge : edge_kind -> 'e -> 's -> 's
method leave_edge : edge_kind -> 'e -> 's -> 's