Shell Programming and Scripting

Hi,

I'm new to the Ash shell so my apologies if this is well known. In normal maths and other shells and languages I've used, the modulo operator always returns a positive remainder. For example see this discussion (first post so I can't hyperlink it):

http://math.stackexchange.com/questions/519845/modulo-of-a-negative-number

The Ash shell (and I am told Bash too) return a negative result. This code run on BusyBox illustrates the problem:

Code:

#!/bin/sh
echo "The result of A % B should be an integer in the range 0 to B-1"
echo "(9-7) % 5 = $(((9-7)%5)) (should be 2)."
echo "(7-9) % 5 = $(((7-9)%5)) (should be 3)."
exit 0

Is this accepted as just a 'feature' of the shells or is it a bug?