A distributed team ships software across many time zones. Each engineer maintains a personal calendar as a sorted list of the intervals when they are already booked. Because the team is fully remote, a meeting can only be scheduled during a window in which every single engineer is free. Any time that hits a single booked block on any calendar cannot be used for a team-wide call.

You are given schedule, a list of engineers, where each engineer is itself a sorted list of busy intervals written as [start, end]. Every engineer's own busy intervals are disjoint (never overlap), but engineers are never told about their teammates' bookings, so their busy windows do overlap across the group.

Return the list of common free intervals: all maximal time windows during which every engineer is simultaneously free, sorted by start time. A window [s, e] means every moment strictly between s and e is free for the whole team; being free at a single instant (for example a meeting that ends exactly when another begins) does not count as a usable gap, because handoff time is needed.