diff --git a/sw/lib/ocaml/extXml.ml b/sw/lib/ocaml/extXml.ml
index 1658d43c1d..246d72a160 100644
--- a/sw/lib/ocaml/extXml.ml
+++ b/sw/lib/ocaml/extXml.ml
@@ -178,12 +178,26 @@ let subst_attrib = fun attrib value xml ->
 let subst_child = fun ?(select= fun _ -> true) t x xml ->
   match xml with
     Xml.Element (tag, attrs, children) ->
-      Xml.Element (tag,
-		   attrs,
-		   List.map (fun xml -> if tag_is xml t && select xml then x else xml) children)
+      let found = ref false in
+      let new_children = 
+	List.map
+	  (fun xml -> if tag_is xml t && select xml then (found := true; x) else xml)
+	  children in
+      if !found then
+        Xml.Element (tag, attrs, new_children)
+      else
+	raise Not_found
   | Xml.PCData _ -> xml
 
 
+let subst_or_add_child = fun t x xml ->
+  try subst_child t x xml with Not_found ->
+    match xml with
+      Xml.Element (tag, attrs, children) ->
+	Xml.Element (tag, attrs, x::children)
+    |  Xml.PCData _ -> xml
+
+
 let remove_child = fun ?(select= fun _ -> true) t xml ->
   match xml with
     Xml.Element (tag, attrs, children) ->
diff --git a/sw/lib/ocaml/extXml.mli b/sw/lib/ocaml/extXml.mli
index a6eb90443e..51baa6150d 100644
--- a/sw/lib/ocaml/extXml.mli
+++ b/sw/lib/ocaml/extXml.mli
@@ -58,11 +58,14 @@ val subst_attrib : string -> string -> Xml.xml -> Xml.xml
 
 val subst_child :
     ?select:(Xml.xml -> bool) -> string -> Xml.xml -> Xml.xml -> Xml.xml
-(** [subst_child ?select child_tag new_child xml] *)
+(** [subst_child ?select child_tag new_child xml] May raise Not_found *)
+
+val subst_or_add_child : string -> Xml.xml -> Xml.xml -> Xml.xml
+(** [subst_or_add_child child_tag new_child xml] *)
 
 val remove_child :
     ?select:(Xml.xml -> bool) -> string -> Xml.xml -> Xml.xml
-(** [delete_child ?select child_tag xml] *)
+(** [delete_child ?select child_tag xml] Returns [xml] if not found *)
 
 val parse_file : ?noprovedtd:bool -> string -> Xml.xml
 (** Identical to Xml.parse_file with Failure exceptions. [nodtdprove] default is false. *)