What is the central role of «al-qāʾim» in «Zayd huwa al-qāʾim»?

The Separating Pronoun (Ḍamīr al-Faṣl): Restriction, Emphasis and Predication

• Replacing Zayd as the pronoun’s antecedent
• The predicate attributing standing
• A preposition associated with the subject
• A particle negating the sentence