use structnum instead of label when using stashed structure

CHANGELOG.md

@@ -0,0 +1,12 @@
+# Changelog
+All notable changes to the `tagpdf` package since the
+2021-04-22 will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+this project uses date-based 'snapshot' version identifiers.
+## [Unreleased]
+
+### Changed
+
+- Ulrike Fischer, 2024-11-29
+  luamml-structelemwriter.lua: use structnum instead of label when stashing. 

luamml-structelemwriter.lua

@@ -81,10 +81,8 @@ local function write_elem(tree, stash)
   table.sort(attrs)
 
   if stash then
-    stash_cnt = stash_cnt + 1
-    stash = '__luamml_stashed_' .. stash_cnt
-    tree[':struct'] = stash
-    stash = ', stash, label = ' .. stash
+    tree[':structnum'] = get_ltx().tag.get_struct_num_next() 
+    stash = ', stash, '
   end
 
   local attr_flag = i ~= 0 and ', attribute=' .. attributes[table.concat(attrs)]