Can't roll / pop from nested arrays

Somewhat rlated to the `rock x at y` problem that was recently fixed, `roll x at y` gives a parsing error
```
value = 1
pos = 0
rock arr at pos value
say arr   (this works now: [ [ 1 ] ])
roll arr at pos into res   (this line causes an error)
say res   (expected: 1)
```