دَخَلَ زَيْدٌ فَخَرَجَ عَمْرٌ، أَوْ ثُمَّ خَرَجَ عَمْرٌ
Zayd entered, then ʿAmr departed—immediately or after an interval according to the particle.
The contrast demonstrates that faʾ and thumma do not merely coordinate. Their conventional meanings introduce an additional relation of succession or delay. This helps explain why wāw needs an independent contextual reason to group two clauses.
It does not follow that faʾ or thumma can join arbitrary statements. Each has conditions appropriate to its meaning; the point is that their specialized meanings themselves provide a stronger semantic direction.