Human–computer interaction

It is the late 1930's in Spain, during the Spanish Civil War. Robert Jordan lies in the forest. He sees a mill in the distance. An old man, Anselmo, points out the bridge, and says that the mill and the roadmender's hut hold enemy posts. Robert Jordan is tall and thin, in peasant clothes, with fair hair. They take a steep climb. Anselmo is an experienced climber and guide, and Robert Jordan knows this is important when traveling behind enemy lines. He has been sent to blow up the bridge. General Golz explained that the explosion must occur only when the attack has begun, so that the enemy cannot repair it; the date and time is subject to change due to forces out of his control. The enemy moves trucks, tanks, and artillery via the bridge. An aerial bombardment will precede the attack.