/test.mp3:
$ fq -i -d mp3 . /test.mp3
mp3> .headers | ., tovalue, type, length?
    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|0123456789abcdef|.headers: [1]
0x00|49 44 33 04 00 00 00 00 00 23 54 53 53 45 00 00|ID3......#TSSE..|  [0]: {} (id3v2)
*   |until 0x2c.7 (45)                              |                |
[
  {
    "flags": {
      "experimental_indicator": false,
      "extended_header": false,
      "unsynchronisation": false,
      "unused": 0
    },
    "frames": [
      {
        "flags": {
          "compression": false,
          "data_length_indicator": false,
          "encryption": false,
          "file_alter_preservation": false,
          "grouping_identity": false,
          "read_only": false,
          "tag_alter_preservation": false,
          "unsync": false,
          "unused0": 0,
          "unused1": 0,
          "unused2": 0
        },
        "id": "TSSE",
        "size": 15,
        "text": "Lavf58.45.100",
        "text_encoding": 3
      }
    ],
    "magic": "ID3",
    "padding": null,
    "revision": 0,
    "size": 35,
    "version": 4
  }
]
"array"
1
mp3> .headers[0] | ., type, length?
    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|0123456789abcdef|.headers[0]: {} (id3v2)
0x00|49 44 33                                       |ID3             |  magic: "ID3" (Correct)
0x00|         04                                    |   .            |  version: 4
0x00|            00                                 |    .           |  revision: 0
0x00|               00                              |     .          |  flags: {}
0x00|                  00 00 00 23                  |      ...#      |  size: 35
0x00|                              54 53 53 45 00 00|          TSSE..|  frames: [1]
0x10|00 0f 00 00 03 4c 61 76 66 35 38 2e 34 35 2e 31|.....Lavf58.45.1|
0x20|30 30 00                                       |00.             |
0x20|         00 00 00 00 00 00 00 00 00 00         |   ..........   |  padding: Correct (none) (zero padding)
"object"
7
mp3> .headers[-1000] | ., type, length?
null
"null"
0
mp3> .headers[1000] | ., type, length?
null
"null"
0
mp3> .headers[1:3] | ., type, length?
[]
"array"
0
mp3> .headers[0:-1] | ., type, length?
[]
"array"
0
mp3> .headers[-1000:2000] | ., type, length?
[
  {
    "flags": {
      "experimental_indicator": false,
      "extended_header": false,
      "unsynchronisation": false,
      "unused": 0
    },
    "frames": [
      {
        "flags": {
          "compression": false,
          "data_length_indicator": false,
          "encryption": false,
          "file_alter_preservation": false,
          "grouping_identity": false,
          "read_only": false,
          "tag_alter_preservation": false,
          "unsync": false,
          "unused0": 0,
          "unused1": 0,
          "unused2": 0
        },
        "id": "TSSE",
        "size": 15,
        "text": "Lavf58.45.100",
        "text_encoding": 3
      }
    ],
    "magic": "ID3",
    "padding": null,
    "revision": 0,
    "size": 35,
    "version": 4
  }
]
"array"
1
mp3> .headers["test"] | ., type, length?
error: expected an object with key "test" but got: array
mp3> [.headers[]] | type, length?
"array"
1
mp3> .headers | keys
[
  0
]
mp3> .headers | has("a")
error: cannot check whether array has a key: a
mp3> .headers | has(0)
true
mp3> .headers | type
"array"
mp3> .headers | tonumber
error: tonumber cannot be applied to: array
mp3> .headers | tostring
error: tostring cannot be applied to: array
mp3> .headers + ""
error: cannot add: array ([{"flags":{"experimental_ ...]) and string ("")
mp3> .headers + 1
error: cannot add: array ([{"flags":{"experimental_ ...]) and number (1)
mp3> .headers._start | ., type, length?
0
"number"
0
mp3> .headers._stop | ., type, length?
360
"number"
360
mp3> .headers._len | ., type, length?
360
"number"
360
mp3> .headers._name | ., type, length?
"headers"
"string"
7
mp3> .headers._symbol | ., type, length?
""
"string"
0
mp3> .headers._description | ., type, length?
""
"string"
0
mp3> .headers._path | ., type, length?
[
  "headers"
]
"array"
1
mp3> .headers._bits | ., type, length?
    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|                |
0x00|49 44 33 04 00 00 00 00 00 23 54 53 53 45 00 00|ID3......#TSSE..|
<360 bits>
"buffer"
360
mp3> .headers._bytes | ., type, length?
    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|                |
0x00|49 44 33 04 00 00 00 00 00 23 54 53 53 45 00 00|ID3......#TSSE..|
<45 bytes>
"buffer"
45
mp3> .headers._error | ., type, length?
null
"null"
0
mp3> .headers._unknown | ., type, length?
false
"boolean"
mp3> .headers.a = 1
error: cannot update key a for array
mp3> .headers[0] = 1
error: cannot update key 0 for array
mp3> .headers.a |= empty
error: expected an object with key "a" but got: array
mp3> .headers[0] |= empty
error: cannot update key 0 for array
mp3> .headers | setpath(["a"]; 1)
error: cannot update key a for array
mp3> .headers | setpath([0]; 1)
error: cannot update key 0 for array
mp3> .headers | delpaths([["a"]])
error: cannot update key a for array
mp3> .headers | delpaths([[0]])
error: cannot update key 0 for array
mp3> ^D