Artists in CrimeOne of Ngaio Marshs most famous murder mysteries, which introduces Inspector Alleyn to his future wife, the irrepressible Agatha Troy. It started as a student exercise, the knife under the drape, the models pose chalked in place. But before Agatha Troy, artist and instructor, returns to the class, the pose has been re enacted in earnest: the model is dead, fixed for ever in one of the most dramatic poses Troy has ever seen. Its a difficult case for