Affirmed and negated sides of lā

Qasr by Coordination, Negation–Exception, and Innamā

زَيْدٌ قَائِمٌ لَا قَاعِدٌ
Zayd is standing, not sitting.

With coordinating lā the first description is affirmed, while the second is denied. This differs from prohibitive and ordinary negative lā. Read the construction as a contrast between specified alternatives rather than a universal denial.