735 ILCS 5/6-136: Pleadings.

The pleadings following the filing of the petition and the proceedings thereon shall be the same as in ordinary civil actions, but no matters shall be pleaded or presented which were or might have been denied in such action of ejectment. The defendant may plead a recovery by such defendant, or any other person, of the same premises, or of part thereof, subsequent to the verdict of the jury if tried by a jury, or to the finding of the court if tried without a jury, in such action of ejectment, in bar or in mitigation of the damages claimed by the plaintiff.