What is the function of lā in fa-lā taqhar?

Supplementary Practice 18: Rhetorical Analysis of Sūrat al-Ḍuḥā and Sūrat al-Sharḥ

• Prohibitive lā, which puts the imperfect in the jussive
• Absolute-negation lā with taqhar as its noun
• An oath particle making al-yatīm genitive
• A purpose lām forming an oath response