$ fq -d matroska 'matroska_path(".Segment.Tracks[0].TrackEntry[0].CodecID")' /avc.mkv
     |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|0123456789abcdef|.elements[1].elements[3].elements[1].elements[4]{}:
0x150|      86                                       |  .             |  id: "CodecID" (0x86) (An ID corresponding to the codec, see the  for more info.)
     |                                               |                |  type: "string" (3)
0x150|         8f                                    |   .            |  size: 15
0x150|            56 5f 4d 50 45 47 34 2f 49 53 4f 2f|    V_MPEG4/ISO/|  value: "V_MPEG4/ISO/AVC"
0x160|41 56 43                                       |AVC             |
$ fq -d matroska 'matroska_path(".Segment.Tracks[0].TrackEntry[0].CodecID") | matroska_path' /avc.mkv
".Segment.Tracks.TrackEntry.CodecID"
$ fq -d matroska 'matroska_path(matroska_path(".Segment.Tracks[0].TrackEntry[0].CodecID"))' /avc.mkv
".Segment.Tracks.TrackEntry.CodecID"
$ fq -n '"a" | raw | matroska_path(".Segment")'
exitcode: 5
stderr:
error: not matroska format
$ fq -n '1 | matroska_path(".Segment")'
exitcode: 5
stderr:
error: expected decode value but got: number (1)