>I must be totally not undertanding the help:
>
>

>Null values persist through logical expressions in most cases. The following table describes the behavior of null values in logical expressions.
>
>Logical expression  Result if x=TRUE  Result if x=FALSE  Result if x=.NULL.
>x AND .NULL.            .NULL.             FALSE               .NULL.
>x OR .NULL.              TRUE              .NULL.              .NULL.
>NOT x                   FALSE              TRUE                .NULL.
>
>

This table just shows how the shorthand evaluation works - if the value of the logical expression can be decided from the first operand, then the second isn't evaluated.