Update [KitBa] to use [disjoint_union_monoid].

src/KitBa.ml

@@ -4,7 +4,7 @@
    [IllFormed x] is raised if the atom [x] occurs twice in a binding
    position. *)
 
-exception IllFormed of Atom.t
+exception IllFormed = Atom.Set.NonDisjointUnion
 
 type env = unit
 
@@ -16,18 +16,8 @@ class ['self] reduce = object (_ : 'self)
   method private visit_'fn () _x =
     Atom.Set.empty
 
-  (* TEMPORARY could move this monoid to [Atom] *)
   (* The monoid of sets of atoms, equipped with disjoint union, is used. *)
-  method zero =
-    Atom.Set.empty
-  method plus xs ys =
-    match Atom.Set.choose (Atom.Set.inter xs ys) with
-    | exception Not_found ->
-        (* The intersection of [xs] and [ys] is empty; good.
-           Compute their disjoint union. *)
-        Atom.Set.union xs ys
-    | x ->
-        raise (IllFormed x)
+  inherit [_] Atom.Set.disjoint_union_monoid
 
   (* The atom [x] is added to the set of bound atoms when its scope is
      exited. *)