Assuming an original mp4 with the following `udta` block:

```
[moov]
  ....
  [udta] size=8+69
    [.mak] size=8+9
    [.mod] size=8+12

```

I would like to add the following `udta` block **payload only** to the original's `udta` block:

```
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

In order to get the following `udta` block result:

```
[moov]
  ....
  [udta] size=8+69
    [.mak] size=8+9
    [.mod] size=8+12
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

The following command was issued:

```
mp4extract moov/udta --payload original.mp4 udta.payload
mp4edit --insert moov/udta original.mp4 edited.mp4
```

with the result: 

```
[moov]
  ....
  [udta] size=8+69
    // [.mak] and [.mod] have been deleted
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

At first glance, it feels like mp4edit --insert behave like a "replace" box operation rather than append/insert operation.
 
What do you recommend to get both `udta` block merged ?
mp4edit --insert: Support merging 2 same type boxes
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : mp4edit --insert: Support merging 2 same type boxes
Issue date:

--- start body ---
Assuming an original mp4 with the following `udta` block:

```
[moov]
  ....
  [udta] size=8+69
    [.mak] size=8+9
    [.mod] size=8+12

```

I would like to add the following `udta` block **payload only** to the original's `udta` block:

```
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

In order to get the following `udta` block result:

```
[moov]
  ....
  [udta] size=8+69
    [.mak] size=8+9
    [.mod] size=8+12
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

The following command was issued:

```
mp4extract moov/udta --payload original.mp4 udta.payload
mp4edit --insert moov/udta original.mp4 edited.mp4
```

with the result: 

```
[moov]
  ....
  [udta] size=8+69
    // [.mak] and [.mod] have been deleted
    [foobar] size=...
    [foobar2] size= ...
    [foobar3] size=...
```

At first glance, it feels like mp4edit --insert behave like a "replace" box operation rather than append/insert operation.
 
What do you recommend to get both `udta` block merged ?
--- end body ---
