Look. Out of the distance, over the baked and burnished plain, along the rattlesnake trail that winds past red, uprearing buttes, comes the stagecoach. And here, in the foreground, behind a rock that tops a rise, we see a group of men. Who they are, we do not know. We do know, from their stubbled faces, their greasy waistcoats, their crushed and begrimed headgear, that they are up to no good. Their very horses are mud-coloured and nameless. It is a matter of certain prediction that when the coach draws abreast of the rock, these men will rob it. We do not yet know how the robbery will proceed - whether a boulder will roll downhill into the path of the horses, whether a shot will tumble the guard from his perch, or whether, after a prolonged chase, the most athletic bandit will swing on the harness between the lead horses and bring the equipage to a standstill. What we are assured of is, that given these elements - the coach in the distance, the men lurking behind the rock - the narrative must take a certain turn; elements of structure enable us to predict a course of events.