+let rec merge_list t l1 l2 =
+ match l1,l2 with
+ | [],l2 -> l2
+ | l1,[] -> l1
+ | h1::ll1, h2::ll2 -> if (comp_node t h2 h1) then h1:: (merge_list t ll1 l2)
+ else if (comp_node t h1 h2) then h2:: (merge_list t l1 ll2)
+ else h1::(merge_list t ll1 ll2)
+