[CSI] negative indexes

[Mikhail Nesterenko]

CSI students,

There was a question last time whether negative indexes are allowed in
the array. That is, if this

    a[-5]

is legal. Indeed it is legal. However, negative index still needs to
point to a valid element of the array. 

In general, indexing operation a[i] is equivalent to *(a+i) in pointer
arithmetic.

Hence,  a[-5] is equivalent to *(a-5).

Thanks,
-- 
Mikhail