diff --git a/luamml-tex.lua b/luamml-tex.lua
index 852c93a..6f08ec5 100644
--- a/luamml-tex.lua
+++ b/luamml-tex.lua
@@ -7,6 +7,8 @@ local mappings = require'luamml-legacy-mappings'
 local write_xml = require'luamml-xmlwriter'
 local write_struct = require'luamml-structelemwriter'
 
+local filename_token = token.create'l__luamml_filename_tl'
+
 local properties = node.get_properties_table()
 
 local funcid = luatexbase.new_luafunction'RegisterFamilyMapping'
@@ -39,8 +41,18 @@ end
 local mlist_buffer
 local mlist_result, mlist_display
 
+local undefined_cmd = token.command_id'undefined_cs'
+local call_cmd = token.command_id'call'
+
 local function save_result(xml, display)
   mlist_result, mlist_display = xml, display
+  if filename_token.command ~= undefined_cmd then
+    assert(filename_token.command == call_cmd)
+    token.put_next(filename_token)
+    assert(io.open(token.scan_string(), 'w'))
+      :write(write_xml(make_root({[0] = 'mrow', xml}, display and 0 or 2)) .. '\n')
+      :close()
+  end
   if tex.count.tracingmathml > 1 then
     -- Here xml gets wrapped in an mrow to avoid modifying it.
     texio.write_nl(write_xml(make_root({[0] = 'mrow', xml}, display and 0 or 2)) .. '\n')
diff --git a/luamml.sty b/luamml.sty
index 10e07ec..c14f68e 100644
--- a/luamml.sty
+++ b/luamml.sty
@@ -2,24 +2,28 @@
   {Feel free to add a description here}
 
 \int_new:N \l__luamml_flag_int
+\tl_new:N \l__luamml_filename_tl
 \int_new:N \tracingmathml
 \lua_now:n { require'luamml-tex' }
 
-\cs_new:Nn \luamml_flag_save: {
+\cs_new:Npn \luamml_flag_save: {
   \int_set:Nn \l__luamml_flag_int { 2 }
 }
-\cs_new:Nn \luamml_flag_ignore: {
+\cs_new:Npn \luamml_flag_ignore: {
   \int_set:Nn \l__luamml_flag_int { 3 }
 }
-\cs_new:Nn \luamml_flag_alignment_left: {
+\cs_new:Npn \luamml_flag_alignment_left: {
   \int_set:Nn \l__luamml_flag_int { 7 }
 }
-\cs_new:Nn \luamml_flag_alignment_right: {
+\cs_new:Npn \luamml_flag_alignment_right: {
   \int_set:Nn \l__luamml_flag_int { 6 }
 }
-\cs_new:Nn \luamml_flag_structelem: {
+\cs_new:Npn \luamml_flag_structelem: {
   \int_set:Nn \l__luamml_flag_int { 8 }
 }
+\cs_new:Npn \luamml_set_filename:n {
+  \tl_set:Nn \l__luamml_filename_tl
+}
 
 \cs_new:Npn \__luamml_patch_package:nn #1 #2 {
   \@ifpackageloaded {#1} {#2} {
diff --git a/test_structelem.tex b/test_structelem.tex
new file mode 100644
index 0000000..5fd6e33
--- /dev/null
+++ b/test_structelem.tex
@@ -0,0 +1,34 @@
+% !Mode:: "TeX:DE:UTF-8:Main"
+\PassOptionsToPackage{enable-debug,check-declarations}{expl3}
+\RequirePackage{pdfmanagement-testphase}
+\DeclareDocumentMetadata{uncompress,pdfversion=2.0}
+\RequirePackage{expl3}
+\documentclass{article}
+\usepackage{unicode-math}
+\usepackage{tagpdf}
+\tagpdfsetup{activate-all,interwordspace=true,}
+\usepackage{amsmath}
+\usepackage{luamml}
+% \RegisterFamilyMapping\symsymbols{oms}
+% \RegisterFamilyMapping\symletters{oml}
+% \RegisterFamilyMapping\symlargesymbols{omx}
+\ExplSyntaxOn
+
+\luamml_flag_structelem:
+\int_new:N \g__my_mathml_int
+\luamml_set_filename:n {
+  \immediateassignment \int_gincr:N \g__my_mathml_int
+  \jobname -formula- \int_use:N \g__my_mathml_int .xml
+}
+\ExplSyntaxOff
+\begin{document}
+\tagstructbegin{tag=Document}
+\tagstructbegin{tag=Formula}
+\[
+  ax^2+bx+c=0,\quad
+  \int_1^n f(x) = \sum_{i=1}^m x^4 dx
+\]
+\tagstructend
+\tagstructend
+
+\end{document}