Variations on the Canon exercise

This exercise is for two or more players, where one player is the leader. The leader improvises a melody, which the followers play at any time-interval agreed upon, like a canon. The leader trains to imagine the auditive result of his/her actions, while the followers have to be fast enough to correctly recognize what the leader plays, and perform this on their instruments. A certain level of freedom can be introduced for the imitation: variation or even contrast can be allowed.