diff --git a/src/stdune/map.ml b/src/stdune/map.ml
index 329178bd..efc46028 100644
--- a/src/stdune/map.ml
+++ b/src/stdune/map.ml
@@ -116,4 +116,7 @@ module Make(Key : Comparable.S) : S with type key = Key.t = struct
       | None      -> assert false
       | Some data -> f key data)
   let filter_map t ~f = filter_mapi t ~f:(fun _ x -> f x)
+
+  let superpose a b =
+    union a b ~f:(fun _ _ y -> Some y)
 end
diff --git a/src/stdune/map_intf.ml b/src/stdune/map_intf.ml
index 9c6fa22e..9677ef5a 100644
--- a/src/stdune/map_intf.ml
+++ b/src/stdune/map_intf.ml
@@ -21,6 +21,10 @@ module type S = sig
     -> f:(key -> 'a -> 'a -> 'a option)
     -> 'a t
 
+  (** [superpose a b] is [b] augmented with bindings of [a] that are
+      not in [b]. *)
+  val superpose : 'a t -> 'a t -> 'a t
+
   val compare : 'a t -> 'a t -> compare:('a -> 'a -> Ordering.t) -> Ordering.t
   val equal   : 'a t -> 'a t -> equal:('a -> 'a -> bool) -> bool
 
diff --git a/src/stdune/univ_map.ml b/src/stdune/univ_map.ml
index 9bdb0f20..a17795be 100644
--- a/src/stdune/univ_map.ml
+++ b/src/stdune/univ_map.ml
@@ -72,3 +72,5 @@ let find_exn t key =
     Eq.cast eq v
 
 let singleton key v = Int.Map.singleton (Key.id key) (Binding.T (key, v))
+
+let superpose = Int.Map.superpose
diff --git a/src/stdune/univ_map.mli b/src/stdune/univ_map.mli
index 6cea4955..03d20724 100644
--- a/src/stdune/univ_map.mli
+++ b/src/stdune/univ_map.mli
@@ -18,3 +18,7 @@ val remove   : t -> 'a Key.t -> t
 val find     : t -> 'a Key.t -> 'a option
 val find_exn : t -> 'a Key.t -> 'a
 val singleton : 'a Key.t -> 'a -> t
+
+(** [superpose a b] is [b] augmented with bindings of [a] that are not
+    in [b]. *)
+val superpose : t -> t -> t