class virtual ['a] ograph :
object ('o)
  method virtual empty : 'o

  method virtual add_node : 'a -> 'o
  method virtual del_node : 'a -> 'o

  method virtual add_arc : 'a * 'a -> 'o
  method virtual del_arc : 'a * 'a -> 'o


  method virtual nodes : 'a Oset.oset
  method virtual predecessors : 'a -> 'a Oset.oset
  method virtual successors : 'a -> 'a Oset.oset

  method virtual ancestors : 'a Oset.oset -> 'a Oset.oset
  method virtual brothers : 'a -> 'a Oset.oset
  method virtual children : 'a Oset.oset -> 'a Oset.oset

  method mydebug : ('a * 'a list) list
end