Using concat with lots of files halts execution

Application Development
1

I’m trying to concatenate multiple mp4 files. My pipeline seems to work ok until I reach a certain number of files. I can’t remember how many, but I used ChatGPT and Claude to help identify where the issue may be and it was suggested to add queues between the demux and the concat. That did help, but only a little.

With the files I’m testing (All approx 10MB in size), it appears that 35 files is the point at which my pipeline basically halts, but it works fine with 34 files. Here is the last few lines of output from GStreamer:

Redistribute latency...98.4 %)
Redistribute latency...
Redistribute latency...
Redistribute latency...
Redistribute latency...99.2 %)
Redistribute latency...
Redistribute latency...
Redistribute latency...
Redistribute latency...
Redistribute latency...99.0 %)
Redistribute latency...
Redistribute latency...
Redistribute latency...
Redistribute latency...
Redistribute latency...
0:00:51.3 / 0:00:51.4 (99.9 %)

It sits here forever. With 34 files, this pipeline finishes almost instantly. The 35th file opens and plays fine, so I don’t think it’s corrupt. I’ve ran this with debug level 3 and I don’t know that I see anything obvious (see below):

Use Windows high-resolution clock, precision: 1 ms
Setting pipeline to PAUSED ...
0:00:00.123267700       8164      30556 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux34> atom has bogus size 18446744073709551615
0:00:00.123293700       8164      33456 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux33> atom has bogus size 18446744073709551615
0:00:00.123490600       8164        484 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux32> atom has bogus size 18446744073709551615
0:00:00.123858400       8164      30140 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux31> atom has bogus size 18446744073709551615
0:00:00.123869100       8164       9528 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux30> atom has bogus size 18446744073709551615
0:00:00.123977400       8164      30176 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux29> atom has bogus size 18446744073709551615
0:00:00.124119400       8164      26792 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux28> atom has bogus size 18446744073709551615
0:00:00.124288400       8164       7656 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux27> atom has bogus size 18446744073709551615
0:00:00.124483100       8164      11800 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux26> atom has bogus size 18446744073709551615
0:00:00.124633300       8164      28540 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux25> atom has bogus size 18446744073709551615
0:00:00.124809400       8164      27000 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux24> atom has bogus size 18446744073709551615
0:00:00.124893800       8164      30448 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux23> atom has bogus size 18446744073709551615
0:00:00.125072200       8164       7012 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux22> atom has bogus size 18446744073709551615
0:00:00.125246600       8164       8640 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux21> atom has bogus size 18446744073709551615
0:00:00.125415400       8164       6300 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux20> atom has bogus size 18446744073709551615
0:00:00.125546700       8164      34176 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux19> atom has bogus size 18446744073709551615
0:00:00.125855600       8164      27084 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux17> atom has bogus size 18446744073709551615
0:00:00.125857900       8164      25212 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux18> atom has bogus size 18446744073709551615
0:00:00.126022000       8164      34192 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux16> atom has bogus size 18446744073709551615
0:00:00.126178000       8164      33616 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux15> atom has bogus size 18446744073709551615
0:00:00.126355000       8164      17128 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux14> atom has bogus size 18446744073709551615
0:00:00.126513600       8164      25152 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux13> atom has bogus size 18446744073709551615
0:00:00.126690200       8164      28776 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux12> atom has bogus size 18446744073709551615
0:00:00.126855300       8164      20760 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux11> atom has bogus size 18446744073709551615
0:00:00.127010000       8164       1268 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux10> atom has bogus size 18446744073709551615
0:00:00.127163000       8164      29552 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux9> atom has bogus size 18446744073709551615
0:00:00.127340500       8164      20972 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux8> atom has bogus size 18446744073709551615
0:00:00.127498000       8164      32336 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux7> atom has bogus size 18446744073709551615
0:00:00.127628300       8164      17560 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux6> atom has bogus size 18446744073709551615
0:00:00.127807400       8164       3480 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux5> atom has bogus size 18446744073709551615
0:00:00.127958800       8164      30688 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux4> atom has bogus size 18446744073709551615
0:00:00.128159100       8164      24216 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux3> atom has bogus size 18446744073709551615
0:00:00.128327400       8164      13128 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux2> atom has bogus size 18446744073709551615
Pipeline is PREROLLING ...
0:00:00.128444600       8164      22132 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux1> atom has bogus size 18446744073709551615
0:00:00.128631800       8164      21888 WARN                 qtdemux qtdemux.c:560:gst_qtdemux_pull_atom:<demux0> atom has bogus size 18446744073709551615
Redistribute latency...
0:00:00.346235200       8164      21452 FIXME               basesink gstbasesink.c:3399:gst_base_sink_default_event:<filesink0> stream-start event without group-id. Consider implementing group-id handling in the upstream elements
Redistribute latency...
Redistribute latency...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
0:00:00.363595800       8164      21452 FIXME             aggregator gstaggregator.c:1543:gst_aggregator_loop:<mux> Subclass should call gst_aggregator_selected_samples() from its aggregate implementation.

Here is the pipeline:

gst-launch-1.0 concat name=vconcat ! queue ! h264timestamper ! h264parse ! mp4mux name=mux ! filesink location="./test.mp4" ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\7D3F1AE6-E9AA-4E89-939F-DED7E3381040.mp4" ! qtdemux name=demux0 demux0.video_0 ! queue ! h264parse ! vconcat. demux0.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\400EE83E-606A-4A88-BF2A-2133B5FF72B3.mp4" ! qtdemux name=demux1 demux1.video_0 ! queue ! h264parse ! vconcat. demux1.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\00139D31-47A7-4C92-B893-BF355951584F.mp4" ! qtdemux name=demux2 demux2.video_0 ! queue ! h264parse ! vconcat. demux2.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\7C179DA0-2C05-43DB-9A77-50D8F78834DA.mp4" ! qtdemux name=demux3 demux3.video_0 ! queue ! h264parse ! vconcat. demux3.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\907FBD4D-A492-4924-9737-811204689512.mp4" ! qtdemux name=demux4 demux4.video_0 ! queue ! h264parse ! vconcat. demux4.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\8B26ECBF-AE01-4F7F-B8D3-FC466F13199E.mp4" ! qtdemux name=demux5 demux5.video_0 ! queue ! h264parse ! vconcat. demux5.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\41F9ACCF-596E-45AC-9330-065B6C90084C.mp4" ! qtdemux name=demux6 demux6.video_0 ! queue ! h264parse ! vconcat. demux6.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\7ED9E488-4E21-4A30-86AB-669CB1178EC4.mp4" ! qtdemux name=demux7 demux7.video_0 ! queue ! h264parse ! vconcat. demux7.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\994EB291-2FCE-457B-94A6-50085539A8C5.mp4" ! qtdemux name=demux8 demux8.video_0 ! queue ! h264parse ! vconcat. demux8.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\A61C311D-75A7-44EC-97EB-12FAF40EB9DB.mp4" ! qtdemux name=demux9 demux9.video_0 ! queue ! h264parse ! vconcat. demux9.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\48D2A0F3-4B65-423B-8F2C-9EDCCE67E3B5.mp4" ! qtdemux name=demux10 demux10.video_0 ! queue ! h264parse ! vconcat. demux10.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\4EB18333-1C66-4DD0-B249-9ED5D12A085E.mp4" ! qtdemux name=demux11 demux11.video_0 ! queue ! h264parse ! vconcat. demux11.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\FDCE6E2D-A41D-4F37-8ECC-A19EB7AF607F.mp4" ! qtdemux name=demux12 demux12.video_0 ! queue ! h264parse ! vconcat. demux12.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\773ADA79-9A4F-4101-AD2C-FD5E40736C2C.mp4" ! qtdemux name=demux13 demux13.video_0 ! queue ! h264parse ! vconcat. demux13.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\BD9FC25F-065B-492F-BBA7-77ACC3757F9D.mp4" ! qtdemux name=demux14 demux14.video_0 ! queue ! h264parse ! vconcat. demux14.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\6409DE3C-5C44-4078-9B36-A8817052CB19.mp4" ! qtdemux name=demux15 demux15.video_0 ! queue ! h264parse ! vconcat. demux15.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\6D5730D0-2660-4D6F-A041-E5B3AC70EA15.mp4" ! qtdemux name=demux16 demux16.video_0 ! queue ! h264parse ! vconcat. demux16.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\D6120AD0-78F6-46FE-901F-2AD0BD57BB71.mp4" ! qtdemux name=demux17 demux17.video_0 ! queue ! h264parse ! vconcat. demux17.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\C771463C-8FC5-4B4C-AF3C-C838E8283061.mp4" ! qtdemux name=demux18 demux18.video_0 ! queue ! h264parse ! vconcat. demux18.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\297092E1-C02F-4677-AD33-8154A389D735.mp4" ! qtdemux name=demux19 demux19.video_0 ! queue ! h264parse ! vconcat. demux19.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\662EF1E3-19C4-4B1B-B077-86695D844F6D.mp4" ! qtdemux name=demux20 demux20.video_0 ! queue ! h264parse ! vconcat. demux20.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\166B76ED-0E65-422B-818A-23B2F7C275D0\\5C789314-FB20-4ADC-9A2B-17DCF0FC4751.mp4" ! qtdemux name=demux21 demux21.video_0 ! queue ! h264parse ! vconcat. demux21.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\2A0CBE87-CA87-498F-ACF2-E8A07DEDEB48.mp4" ! qtdemux name=demux22 demux22.video_0 ! queue ! h264parse ! vconcat. demux22.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\81294F2F-6226-44DB-A6F8-8C5B5055EE7F.mp4" ! qtdemux name=demux23 demux23.video_0 ! queue ! h264parse ! vconcat. demux23.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\DC3CBD43-6EA0-4F51-899D-2369BD77ACFB.mp4" ! qtdemux name=demux24 demux24.video_0 ! queue ! h264parse ! vconcat. demux24.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\8664DC3B-C46F-483D-B046-6CDA9FE6F7E9.mp4" ! qtdemux name=demux25 demux25.video_0 ! queue ! h264parse ! vconcat. demux25.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\85FF1004-C85A-4527-974E-0737538B9AB2.mp4" ! qtdemux name=demux26 demux26.video_0 ! queue ! h264parse ! vconcat. demux26.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\47A99EC0-7DAD-433C-BC53-30B9FCF4E705.mp4" ! qtdemux name=demux27 demux27.video_0 ! queue ! h264parse ! vconcat. demux27.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\B6F83649-02DD-4610-94C5-AB64EDDB4B62.mp4" ! qtdemux name=demux28 demux28.video_0 ! queue ! h264parse ! vconcat. demux28.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\AF0F18D7-2971-4C49-9B04-E6768E632158.mp4" ! qtdemux name=demux29 demux29.video_0 ! queue ! h264parse ! vconcat. demux29.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\9731D449-7DD2-4599-9897-5CDC51A463E1.mp4" ! qtdemux name=demux30 demux30.video_0 ! queue ! h264parse ! vconcat. demux30.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\577E38FF-4546-4DA0-BF5C-404EC7B21114.mp4" ! qtdemux name=demux31 demux31.video_0 ! queue ! h264parse ! vconcat. demux31.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\81C933FD-F9E4-4D2A-BB31-0FE349373D08.mp4" ! qtdemux name=demux32 demux32.video_0 ! queue ! h264parse ! vconcat. demux32.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\8A4D9595-C7C5-4908-A150-0BA62F432A80.mp4" ! qtdemux name=demux33 demux33.video_0 ! queue ! h264parse ! vconcat. demux33.audio_0 ! queue ! aacparse ! aconcat. ^
filesrc location="D:\\VideoRecording\\Recordings\\F16CFD67-9A2C-463C-A148-E5B683172064\\CA7D0F5E-7FC6-4130-A86F-B9A852EBF611.mp4" ! qtdemux name=demux34 demux34.video_0 ! queue ! h264parse ! vconcat. demux34.audio_0 ! queue ! aacparse ! aconcat. ^
concat name=aconcat ! queue ! aacparse ! mux.

Am I using queues incorrectly? I’ve never felt completely comfortable understanding where to use queues and how to use various queue options, so I’m leaning towards that being my problem.

2

Well, AI suggested increasing queue size and that helped, but I feel like that’s a band-aid and will fail with more and/or larger files.

3

Hi @uler3161

That is one large pipeline! I’d suspect you have run out of resources at file 34? If it happens each time at file 34 I’d feel there is some limit met or an accumulated memory leak?

Have you considered using multifilesrc and rename each file as an index?

multifilesrc allows each file to be played one by one (indexed) to an end pointpoint sink..

gst-launch-1.0 multifilesrc location="<ourfile>.%04d.mp4" index=0 ! qtdemux name=demux demux.video ! .... ! ....  demux.audio ! .... ! ....
4

So, do you think it’s trying to load the data for all files when it starts rather than one at a time? Seems really unnecessary, but it would explain it. I would really prefer to not rename the files, but maybe I might look into that, at least to test your idea.

5

I ended up switching over to splitmuxsrc since my files were created with splitmuxsink. Seems to have solved the issue.

6

Key point (why multifilesrc won’t “just work” with MP4)

multifilesrc simply outputs a continuous byte stream. MP4 is a container with headers (ftyp/moov/moof/mdat) per file, so you cannot safely byte-concat MP4 files and expect qtdemux to treat them as “file-by-file”. It’ll see garbage after the first file ends.

So you need one of these real solutions:

Option A (best CLI): Use GStreamer Editing Services (ges-launch-1.0)

This is the clean “concat clips then render one output” tool.

ges-launch-1.0 -o test.mp4 \
  file:///D:/VideoRecording/Recordings/.../7D3F1AE6-E9AA-4E89-939F-DED7E3381040.mp4 \
  file:///D:/VideoRecording/Recordings/.../400EE83E-606A-4A88-BF2A-2133B5FF72B3.mp4 \
  file:///D:/VideoRecording/Recordings/.../00139D31-47A7-4C92-B893-BF355951584F.mp4

  • It will decode and re-encode as needed (reliable even if clips differ slightly).

  • If you want “mostly copy”/remux, GES isn’t the tool — it’s an editor.

If ges-launch-1.0 isn’t installed on your Windows build, tell me what GStreamer package you installed and I’ll point you at the exact component to add.

Option B (if these MP4s were created by splitmuxsink): use splitmuxsrc

If your recordings are segments of the same recording created by GStreamer’s splitmuxsink, this is the correct “play sequential fragments” source.

gst-launch-1.0 -e ^
  splitmuxsrc location="D:/VideoRecording/Recordings/<yourdir>/%04d.mp4" ^
  ! qtdemux name=demux ^
    demux.video_0 ! queue ! h264parse ! mux. ^
    demux.audio_0 ! queue ! aacparse ! mux. ^
  mp4mux name=mux ! filesink location="test.mp4"

Notes:

  • This does “file-by-file” properly (it understands fragment boundaries), but only if the files match the splitmux format/expectations.

  • If numbering doesn’t start at 0, splitmuxsrc has properties for start index (run gst-inspect-1.0 splitmuxsrc).

Option C (works everywhere, but re-encodes): concatenate at decoded level with concat

This avoids the MP4 container problem by decoding each file first and concatenating raw audio/video.

But in gst-launch, you still have to list each file (no easy loop), and you’ll typically re-encode to get a clean single MP4:

gst-launch-1.0 -e ^
  concat name=v ! videoconvert ! x264enc key-int-max=25 tune=zerolatency ! h264parse ! mux. ^
  concat name=a ! audioconvert ! audioresample ! voaacenc ! aacparse ! mux. ^
  mp4mux name=mux ! filesink location="test.mp4" ^
  filesrc location="D:\...\clip1.mp4" ! qtdemux name=d1 d1.video_0 ! queue ! decodebin ! queue ! v.  d1.audio_0 ! queue ! decodebin ! queue ! a. ^
  filesrc location="D:\...\clip2.mp4" ! qtdemux name=d2 d2.video_0 ! queue ! decodebin ! queue ! v.  d2.audio_0 ! queue ! decodebin ! queue ! a.

This is basically what you wrote, just structured so it’s actually correct for concat.