I have a nested dict `a` and a vector b=[1,2,3].

```
a= {0: {0: [0], 1: [0, 1]}} 

```

I want to add `b` to `a`:

```
a= {0: {0: [0],[1,2,3], 1: [0, 1]}}

```

I tried: `a[0].append([1,2,3])` and `a[0][0].append([1,2,3])`

Neither one is correct.