Does the permissibility of an indefinite mubtadaʾ necessarily negate every other person?

Fronting the Subject: Restriction, Reinforcement and the al-Jurjani–al-Sakkaki Debate

• Yes, because every mubtadaʾ is singular
• Yes, because every indefinite hides lā
• No, because an indefinite can never be mubtadaʾ
• No; syntactic specification and rhetorical exclusivity can differ