What is the subjunctive marker in li-taḍribī?

Imruʾ al-Qays: Address to Fāṭimah, Parting, and the Arrows of the Eyes (Verses 19–22)

• Loss of the nūn from a five-forms imperfect after the purpose-lām
• An obligatory nominative tanwīn on the verb
• Addition of a dual alif to a five-forms verb
• A nūn of emphasis prefixed to the verb