Definition option_cons (A:Type)(k:key)(o:option A)(l:list (key*A)) :=
  match o with
   | Some e => (k,e)::l
   | None => l
  end.Fixpoint map2_l (m : t elt) : t elt'' :=
  match m with
   | nil => nil
   | (k,e)::l => option_cons k (f (Some e) None) (map2_l l)
  end.Fixpoint map2_r (m' : t elt') : t elt'' :=
  match m' with
   | nil => nil
   | (k,e')::l' => option_cons k (f None (Some e')) (map2_r l')
  end.Fixpoint map2 (m : t elt) : t elt' -> t elt'' :=
  match m with
   | nil => map2_r
   | (k,e) :: l =>
      fix map2_aux (m' : t elt') : t elt'' :=
        match m' with
         | nil => map2_l m
         | (k',e') :: l' =>
            match X.compare k k' with
             | LT _ => option_cons k (f (Some e) None) (map2 l m')
             | EQ _ => option_cons k (f (Some e) (Some e')) (map2 l l')
             | GT _ => option_cons k' (f None (Some e')) (map2_aux l')
            end
        end
  end.Notation oee' := (option elt * option elt')%type.Fixpoint combine (m : t elt) : t elt' -> t oee' :=
  match m with
   | nil => map (fun e' => (None,Some e'))
   | (k,e) :: l =>
      fix combine_aux (m':t elt') : list (key * oee') :=
        match m' with
         | nil => map (fun e => (Some e,None)) m
         | (k',e') :: l' =>
            match X.compare k k' with
             | LT _ => (k,(Some e, None))::combine l m'
             | EQ _ => (k,(Some e, Some e'))::combine l l'
             | GT _ => (k',(None,Some e'))::combine_aux l'
            end
        end
  end.Definition fold_right_pair (A B C:Type)(f: A->B->C->C)(l:list (A*B))(i:C) :=
  List.fold_right (fun p => f (fst p) (snd p)) i l.Definition map2_alt m m' :=
  let m0 : t oee' := combine m m' in
  let m1 : t (option elt'') := map (fun p => f (fst p) (snd p)) m0 in
  fold_right_pair (option_cons (A:=elt'')) m1 nil.

Lemma combine_sorted : forall m (Hm : sort (@ltk elt) m) m' (Hm' : sort (@ltk elt') m'), sort (@ltk oee') (combine m m').