Conditions tested?

The examples have two properties which are not specified in the task statement 1. All intervals are of positive integers 2. The intervals are sorted by leftmost point

Is there a test that violates either or both conditions above? If there isn't, here are some suggestions:

[(-5,-4),(-3,1)] -> [(-5,1)]

[(3,5),(4,7),(1,2)] -> [(1,7)]

[(-5,-4),(-3,1),(3,5),(4,7),(1,2)] -> [(-5,7)]