
$num fails with numbers that though large are within script arithmetic range e.g.
Pad UPC 043011001826 to EAN: $num(043011001826,14) (expected 0043011001826)
Compute 043011001826+1: $add(043011001826,1) (expected 43011001827 and got same)
gives
Pad UPC 043011001826 to EAN: 2147483647 (expected 0043011001826)
Compute 043011001826+1: 43011001827 (expected 43011001827 and got same)

$num fails when asked for more than five digits e.g.
Pad catalogue number to six places: $num(1826,6) (expected 001826)
gives
Pad catalogue number to six places: 01826 (expected 001826).