In one recognized parse of “fī ayyi ṣūratin mā shāʾa rakkabaka,” what is mā?

Supplementary Practice 29 — Comparative Rhetoric of Sūrat al-Infiṭār and Sūrat al-Inshiqāq

• Always negative
• Can never be additional
• Additional, while shāʾa qualifies ṣūrah
• The name of inna in a nominal clause