Slicing with dynamic parameters seems to be broken.

As in, this will parse:

``` moon
slice = [ item for item in *items[start, finish] ]
```

But this will not:

``` moon
slice = [ item for item in *items[start + 1, finish - 1] ]
-- Failed to parse:
-- [5] >>    slice = [ item for item in *items[start + 1, finish - 1] ]
```

Not even with parentheses either.
