In what way does the theme of regret appear within the story of Van Horssen and his mentor Verheyen?

Two months after Van Horssen sees his mentor for the last time, he receives word of Verheyen’s death. At Verheyen's funeral, Van Horssen notices that his mentor's amputated leg is not in the casket, as Verheyen had desired. Van Horssen feels deep regret at the fact that his attempts to carry out his teacher’s wishes had been dismissed by Verheyen's family.