Added [Atom.Set.disjoint_union].

src/Atom.ml

@@ -139,6 +139,19 @@ module Set = struct
 
   include Set.Make(Order)
 
+  (* Disjoint union. *)
+
+  exception NonDisjointUnion of atom
+
+  let disjoint_union xs ys =
+    match choose (inter xs ys) with
+    | exception Not_found ->
+        (* The intersection of [xs] and [ys] is empty. Return their union. *)
+        union xs ys
+    | x ->
+        (* The intersection contains [x]. Raise an exception. *)
+        raise (NonDisjointUnion x)
+
   (* Sets of atoms form a monoid under union. *)
 
   class ['z] monoid = object

src/Atom.mli

@@ -28,6 +28,11 @@ val hash: atom -> int
 module Set : sig
   include Set.S with type elt = atom
 
+  (* Disjoint union. *)
+
+  exception NonDisjointUnion of atom
+  val disjoint_union: t -> t -> t
+
   (* Sets of atoms form a monoid under union. *)
 
   class ['z] monoid : object