type 'a prelist = { mutable start : 'a list; mutable current : 'a list}

let start () = { start = []; current = []}

let cons a pl = match pl.current with
  [] -> let l = [a] in pl.current <- l; pl.start <- l
| l -> 
	let l' = [a] in Obj.set_field (Obj.repr l) 1 (Obj.repr l');
        pl.current <- l'

let extract pl = 
  let r = pl.start in 
  pl.current <- []; pl.start <- []; 
  (* to guaranty that we can not mute the list once it has been     
  extracted *)
  r