Should `String.format` allow negative indices?

Python errors out here:

```python
>>> "{-1}".format(1,2)
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
    "{-1}".format(1,2)
    ~~~~~~~~~~~~~^^^^^
KeyError: '-1'
```

Aria happily accepts the input:

```
〉"{-1}".format("a", "b")
b
```

Is this the right thing to do?