let add_depend prop_id fmt =
let f _ =
let msg = Format.flush_str_formatter () in
let dep = { Wpo.dep_pid = prop_id ; Wpo.dep_descr = msg } in
match !stack with
| top::_ -> top.c_depends <- dep::top.c_depends
| _ -> Wp_parameters.fatal "Datalib.Collector: empty stack"
in Format.kfprintf f Format.str_formatter fmt