The mystery novel is a game between author and reader, the goal of which is Find the Villain, and Foul Play is not allowed. The reader is entitled to expect that everything in the mystery relates to the objective. This, then, lays a heavy stricture upon the mystery writer. He may express himself as much as he wants, but he may only do so within the framework of the mystery form -- meaning it must have a bearing on the plot. It must relate to Find the Villain. To work under such a limitation requires artistry and skill. That is what makes the mystery such a demanding art form. The mystery, to the novel, is what the sonnet is to poetry.
(Hillary Waugh, Introduction, How to Write Mysteries, 1989)

Spiele mit strengen Regeln, die ihren Reiz aus dem Umgang mit der Strenge beziehen und aus den Freiheiten, auf die sie verzichten.