let add_dpd graph v1 dpd_kind opt_zone v2 =
try
let edge, old_dpd = find_dpd graph v1 v2 in
let new_dpd = DpdZone.add old_dpd dpd_kind opt_zone in
if not (DpdZone.equal old_dpd new_dpd) then
replace_dpd graph edge new_dpd
with Not_found ->
let new_dpd = DpdZone.make dpd_kind opt_zone in
simple_add_dpd graph v1 new_dpd v2