These would seem to be equivalent but according to the interperter they are not. I have been banging my head with this for along time

```
>>> a = '123456789'
>>> a[0:9]
'123456789'
>>> a[0:9:1]
'123456789'
>>> a[::1]
'123456789'
>>> a[::-1]
'987654321'
>>> a[0:9:-1] #why is this not equivilent to a[::-1]
''
>>>
```