### merge lists of lists

I realize that if I have two lists, say

```
l1 = [1,2,3]
l2 = [4,5,6]
```

then I can join them via

```
l1 + l2
```

i.e. this gives [1,2,3,4,5,6].

I want to know how to do this when my list is not a list of numbers, but a list of matrices. I imagine it would be the same for a list of lists but I don't know how to do that either.