diff --git a/config-lua.lua b/config-lua.lua
index 119e608..22e287b 100644
--- a/config-lua.lua
+++ b/config-lua.lua
@@ -2,3 +2,16 @@ testfiledir  = "testfiles-lua"
 checkengines = {"luatex"}
 stdengine     = "luatex"
 checkruns    = 3
+
+test_types = test_types or {}
+test_types.mml = {
+  test = '.mlt',
+  generated = '.mml',
+  reference = '.mlr',
+  expectation = '.mle',
+  rewrite = function(source, result, engine, errlevels)
+    return os.execute(string.format('tidy -xml -indent -wrap -quiet --output-file "%s" "%s"', result, source))
+  end,
+}
+
+test_order = {'log', 'pdf', 'mml'}
diff --git a/luamml-demo.sty b/luamml-demo.sty
index c68620c..eb4ee9c 100644
--- a/luamml-demo.sty
+++ b/luamml-demo.sty
@@ -36,6 +36,12 @@
     \jobname -formula- \int_use:N \g__luamml_demo_mathml_int .xml
   }
 }
+\DeclareOption{l3build}{
+  \luamml_set_filename:n {
+    \jobname .mml
+  }
+  \luamml_begin_single_file:
+}
 \ProcessOptions\relax
 
 \cs_new_eq:NN \LuaMMLSetFilename \luamml_set_filename:n
diff --git a/luamml-tex.lua b/luamml-tex.lua
index 49f645a..c391a0a 100644
--- a/luamml-tex.lua
+++ b/luamml-tex.lua
@@ -66,6 +66,8 @@ end
 --    Bit 1: Save MathML as a fully converted formula
 --    Bit 0: Save MathML for later usage in startmath node. Ignored for display math.
 
+local out_file
+
 local mlist_result
 
 local undefined_cmd = token.command_id'undefined_cs'
@@ -75,14 +77,18 @@ local labelled_mathml = {}
 
 local function save_result(xml, display, structelem)
   mlist_result = make_root(xml, display and 0 or 2)
-  token.put_next(filename_token)
-  local filename = token.scan_argument()
-  local tracing = tex.count.tracingmathml > 1
-  if filename ~= '' then
-    assert(io.open(filename, 'w'))
-      :write(write_xml(mlist_result, true):sub(2) .. '\n')
-      :close()
+  if out_file then
+    out_file:write(write_xml(mlist_result, true):sub(2) .. '\n')
+  else
+    token.put_next(filename_token)
+    local filename = token.scan_argument()
+    if filename ~= '' then
+      assert(io.open(filename, 'w'))
+        :write(write_xml(mlist_result, true):sub(2) .. '\n')
+        :close()
+    end
   end
+  local tracing = tex.count.tracingmathml > 1
   if tracing then
     texio.write_nl(write_xml(mlist_result) .. '\n')
   end
@@ -159,6 +165,25 @@ lua.get_functions_table()[funcid] = function()
   mlist_result = nil
 end
 
+funcid = luatexbase.new_luafunction'luamml_begin_single_file:'
+token.set_lua('luamml_begin_single_file:', funcid, protected)
+lua.get_functions_table()[funcid] = function()
+  token.put_next(filename_token)
+  local filename = token.scan_argument()
+  if filename ~= '' then
+    out_file = assert(io.open(filename, 'w'))
+  end
+end
+
+funcid = luatexbase.new_luafunction'luamml_end_single_file:'
+token.set_lua('luamml_end_single_file:', funcid, protected)
+lua.get_functions_table()[funcid] = function()
+  if out_file then
+    out_file:close()
+    out_file = nil
+  end
+end
+
 local annotate_context = require'luamml-tex-annotate'
 annotate_context.data.mathml = labelled_mathml
 
diff --git a/luamml.dtx b/luamml.dtx
index e577199..4d6f84e 100644
--- a/luamml.dtx
+++ b/luamml.dtx
@@ -227,6 +227,12 @@
 }
 %</luatex>
 %    \end{macrocode}
+%
+% \begin{macro}{\luamml_begin_single_file:, \luamml_end_single_file:}
+%   Everything between these two commands gets written into the same XML file.
+%   The filename is expanded when \cs{luamml_begin_single_file:} gets executed.
+%
+%   (Implemented in Lua)
 % \end{macro}
 %
 % By default, the flag is set to assume complete formulas.