>Thanks, that was exactly what I was looking for.
>
>Basically, I wasn't sure whether I could omit the parentheses in an expression of the type:
>
>

>(A and B) or (C and D)
>
>or:
>
>(A or B) and (C or D)
>

>
>I now see that, because of precedence, I can omit parentheses in the first case, but not in the second.
>

You can but you shouldn't because with parenthesis expression is much eithier to read and it also works exactly as you intended. :)