This is a scenario that comes up quite a lot when driving on motorways.
Lets say I'm driving in left-most lane (as everyone should unless overtaking), and up ahead there's a car cruising along at a lower speed in the middle lane but not overtaking anyone.
Am I entitled to continue in the left lane and undertake?
If not, I'd have to move to middle lane, then right lane, then back to middle lane and back to left lane when the car has been overtaken.
What if the other car was in the right lane? Would I (and everyone else who comes up behind us) have to just follow them in the right lane indefinitely until they moved into the middle lane?