P sued under 42. U.S.C. 1983 alleging that his arrest violated his constitutional rights. The suit was dismissed because P failed to allege a lack of probable cause. P sought leave to amend, and it was denied. P then brought a second action a year later based on the same arrest. This complaint included an alleged lack of probable cause. The suit was dismissed under res judicata. P appealed. The two complaints are identical in substance except that an averment that the arrest was made without probable cause was included in the second, but not the first.