Intervals - Employee Free Time
Examine this snippet of sweep line code for Employee Free Time. Which line contains a subtle bug that causes incorrect free time intervals?
```python
events.sort(key=lambda x: (x[0], -x[1])) # sort with start events before end events
for time, e_type in events:
if active == 0 and prev is not None and prev < time:
free_times.append([prev, time])
active += e_type
prev = time
```