Handling Array Limit over runs in slice

[sg...@mozilla.com]

Hello,

I really like the slice operator e.g

slice = [item for item in *items[1,5]]

however, suppose items has only 4 elements and further suppose items is backed by a FFI C array. 

Q: How would i throw an error if the user attempts to access elements beyond the bound of items?

Thanks much in advance

Saptarshi