[enhance] xhtml.opa: added a function to update an xhtml attribute by…

… appending a new value to it (usefull for updating class attribute especially)

stdlib/core/xhtml/xhtml.opa

@@ -1155,6 +1155,25 @@ Xhtml =
     end
     aux(x)
 
+  /**
+   * Update (by appending) an attribute to an xhtml node, add it if not already present
+   */
+  update_attribute(name: string, value: string, x:xhtml):xhtml =
+    rec aux(x)=
+    match x : xhtml
+    {fragment=[x]} -> aux(x)
+    {~args namespace=_ tag=_ content=_ specific_attributes=_} as x->
+      args = match find_attr(name,args) with
+             {some=val} ->
+               value = "{val} {value}"
+               l = remove_attr(name,args)
+               [{~name namespace="" ~value}|l]
+             {none} -> [{~name namespace="" ~value}|args]
+      @opensums({x with ~args})
+    _ -> x
+    end
+    aux(x)
+
   /**
    * Set an attribute to an xhtml node. Replace if already_exists
    */