2.2.12.3.1.6 NULL Predicate

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The NULL predicate MUST be true if the item doesn’t have a value for the specified property. Its basic syntax is as follows:

 …property IS NULL…

It has an alternate syntax that reverses its meaning. The alternate syntax is as follows:

 …property IS NOT NULL…
 null-predicate      =      identifier 1*ws "IS" 1*ws ["NOT" 1*ws] "NULL"