Food for thought:-
Scenario - base of controlled airspace 3000'. IFR traffic instructed to fly at 3500' (ATC are required to keep such flights 500' above the base), but descends to 3250'.
Another aircraft in the vicinity, ostensibly flying at 2900', outside and below CAS, inadvertently climbs to 3150'. Vertical separation 100' - but by definition neither aircraft is deemed to have 'bust its level' - that's OK then !!
