The use of the article with a predicate NP follows the normal rules of definiteness. However, since a predicate NP almost always contains new (abstract, indefinite) information about the (already defined) subject, it will only rarely be defined. In violation of the normal rules a superlative degree also usually lacks the article when it serves as predicate NP.
+ agreeing constituent (same principle)
Usually, the article is used anyway when the predicate NP is represented:
- as already known or generally known;
- as (logically or actually) identical with the subject (e.g. as a substantivised participle);
- as a characterising property of the subject (e.g. in definitions).