How do I specify a DimArray over a subset of a dimension?

I thought this would work but it gives an error.

```
Ddim = Dim{:Ddim}(['a','b','c'])
DimArray( 1:2, Ddim(['a','b']) )
```

How do I specify a DimArray over a subset of a dimension?

I thought this would work but it gives an error.

```
Ddim = Dim{:Ddim}(['a','b','c'])
DimArray( 1:2, Ddim(['a','b']) )
```

Create the full array first and index into it.

```
Ddim = Dim{:Ddim}(['a','b','c'])
arr = DimArray(1:3, Ddim)
subarr = arr[At(['a','b'])]
```

I’d like to concatenate several arrays. Something like:

```
cat( [
DimArray( 1:2, Ddim(['a','b']) ),
DimArray( 3:4, Ddim(['c','d']) ),
DimArray( 5:6, Ddim(['e','f']) )
]... ,
dims= :Ddim )
```

```
vcat(
DimArray(1:2, Dim{:Ddim}(['a','b'])),
DimArray(3:4, Dim{:Ddim}(['c','d'])),
DimArray(5:6, Dim{:Ddim}(['e','f']))
)
```

I think this works.

when a dimension is out of order then the dimension elements get dropped on concatenation

```
cat(
DimArray([1 2;3 4], ( Dim{:Ddim}(['a','d']), Dim{:Adim}(['x','y']) ) ),
DimArray([5 6;7 8], ( Dim{:Ddim}(['b','c']), Dim{:Adim}(['x','y']) ) ),
dims=:Ddim )
```

I think it works if the Dimension in the initial Dim arrays are also unordered

To avoid XY problems, maybe give a little more code for context and reasons why you want to do this.