What is the syntactic role of «يضحك» in «جَاءَ زَيْدٌ يَضْحَكُ»?

Supplementary Practice 1: Applied Analysis of Faṣl, Waṣl, Ījāz and Iṭnāb

• A delayed nominal subject
• An oath clause
• A verbal circumstantial clause with an implied subject
• An inserted prayer