In order to permit expressions like ((DIAM GT 200.0) OR (TYPE EQ ’BOX’)), expressions must be able to cope with undefined values. Generally, applying an operator to one or more undefined arguments has an undefined result.

Two exceptions are: the use of the AND operator with a false argument, will result in FALSE, regardless of whether or not the remainder of the arguments are defined; and OR which returns TRUE if any of its arguments is TRUE. For example, consider applying the above expression when the current element is a box. DIAM is undefined; therefore (DIAM GT 200.0) is also undefined. However, (TYPE EQ ’BOX’) is certainly true and so the final result of the whole expression evaluates to TRUE.