How should one handle the governing clause of the opening idhā in az-Zalzalah?

Supplementary Practice 25 — Comparative Rhetoric of Sūrat az-Zalzalah and Sūrat al-ʿĀdiyāt

• Consider recognized analyses involving tuḥaddithu, yaṣduru, or an implied verb
• Declare mā lahā the only permissible response and reject all others
• Treat idhā as a preposition
• Treat idhā and yawmāʾidhin as meaningless additions