The existence operator (?) applies only to a reference to a variable group member. An error occurs if you apply this operator to any other operand. You can use ? to check for the existence of any member in a variable group. In addition, if you use _ALL to refer to all members in the variable group, ? returns the number of members in the group.

The following example yields true (1) if the value of the first message token is a member of the variable group MountDirs; otherwise, it yields false (0):

?MountDirs:_TOKEN1

If MountDirs is a variable group whose members are /dev and /usr, the following constraint allows a message to match only if the first message token is /dev or /usr:

CONSTRAINT (?MountDirs:_TOKEN1)

The following example results in the count of the existing members of MountDirs:

?MountDirs:_ALL