Added [disjoint_union_monoid].

src/Atom.ml

@@ -154,11 +154,18 @@ module Set = struct
 
   (* Sets of atoms form a monoid under union. *)
 
-  class ['z] monoid = object
+  class ['z] union_monoid = object
     method zero: 'z = empty
     method plus: 'z -> 'z -> 'z = union
   end
 
+  (* Sets of atoms form a monoid under disjoint union. *)
+
+  class ['z] disjoint_union_monoid = object
+    method zero: 'z = empty
+    method plus: 'z -> 'z -> 'z = disjoint_union
+  end
+
   (* These printing functions should be used for debugging purposes only. *)
 
   let print_to_scratch xs =

src/Atom.mli

@@ -33,9 +33,15 @@ module Set : sig
   exception NonDisjointUnion of atom
   val disjoint_union: t -> t -> t
 
-  (* Sets of atoms form a monoid under union. *)
+  (* Sets of atoms form monoids under union and disjoint union. *)
 
-  class ['z] monoid : object
+  class ['z] union_monoid : object
+    constraint 'z = t
+    method zero: 'z
+    method plus: 'z -> 'z -> 'z
+  end
+
+  class ['z] disjoint_union_monoid : object
     constraint 'z = t
     method zero: 'z
     method plus: 'z -> 'z -> 'z