I have a list of lists: 

```
x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

```

I want to prepend a string 'a' to each list in the list such that x becomes:

```
[['a', 1, 2, 3], ['a', 4, 5, 6], ['a', 7, 8, 9]]

```

What is the most pythonic way of achieving this?

What I've got so far:

```
[l.insert(0, 'a') for l in x]

```