air battle before normal battle:
for air battles, planes can NOT retreat in the last round (if it is the only one or the tenth..), although property "Air Battle Attackers/Defenders Can Retreat" is true.
planes in airbattle should also be able to retreat in the last round independent of round number (if 1 or 10..).
reason is, even when airbattle is over after the last round, then the players should not be forced to continue the normal battle with the planes.
specially when the attacker sends no ground units in.