And you never break main doing that? MR A could be fine, MR B could be fine, but A+B can be broken. It's rare, but it does happen. It's not common enough to

And you never break main doing that? MR A could be fine, MR B could be fine, but A+B can be broken. It's rare, but it does happen. It's not common enough to fix if the alternative is slowing down merges. But you can have your cake and eat it too -- merge trains allow rapid merging and ensure that the combination gets tested before merging.