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The Matter of the McAlister Murders
Whit Knowlton, a retired lawyer in his eighties, loved to reminisce about his trials. His memory was shaky, however, and he sometimes had to let his listeners fill in blank spots in his narratives.
One lazy Sunday afternoon in the summer, Knowlton was sitting in his well-stocked library swapping stories with Thomas P. Stanwick. The amateur logician always enjoyed his visits with Knowlton. "The McAlister murders in Baltimore hit the papers in the winter of 1953," said the lawyer. "Four men were arrested for the murders within a week, and I was called in to assist the district attorney.
"All four were indicted, but evidence brought out in the course of the trial proved that only two of them were guilty. Our problem was to find out which two they were."
"How useful was their testimony?" asked Stanwick.
"Oh, useful enough, as it turned out," Knowlton replied. "The first defendant, Addler, said that either the second defendant, Bryan, was guilty, or the fourth defendant, Derrick, was innocnet. Bryan said that he was innocent, and that either Adler or Collins, the third defendant, was guilty.
"Collins said that Bryan and Derrick were not both guilty. Derrick, after a long refusal to speak at all, said that Collins was innocent if and only if Addler was innocent."
"were any of their statements proven true?" Stanwick asked.
"Well, yes. Other evidence proved that the two guilty ones were lying and the two innocent ones were telling the truth. Unfortunately, I don't quite remember anymore who the guilty ones were."
Stanwick smiled and fumbled in his pocket for his pipe.
"This case is a treat, Whit," he said, "for it's not too hard to deduce who the guilty ones are."

Who are the guilty ones?