Options:

• vertices - list of vertex indices (not vertex ids!) or :all (default)
• state - initial state
• direction - direction of edges connected to current vertex
  • :forward - out-edges
  • :reverse - in-edges
  • :both - all edges
• process_vertex_fun callback for processing vertex
• process_edge_fun - callback for processing edges
• edge_process_order - whether the (dfs tree) edge processing will be made

  - before (:preorder, default)
  - after (:postorder)
  - before and after (:both)