The "nationality" mysteries had the unusual feature of a "Challenge to the Reader" just before the ending is revealed—the novel breaks the fourth wall and speaks directly to the reader. This is the final instance of the challenge. "For years now I have been challenging my readers to solve my cases by the exercise of close observation, the application of logic to the winnowed facts, and a final correlation of the individual conclusions... Technically there are no snags. The facts are all here at this point... Can you put them together and logically place your finger on the one and only possible murderer?"