Create a prop and move it 1 unit per frame.

proptest.cpp

class script {
  /* We don't actually use the scene object in this script, but most will. */
  scene@ g;
  prop@ p;

  int frame;
  int frame_cp;

  script() {
    @g = get_scene();
    @p = null;

    frame = 0;
  }

  void step(int entities) {
    if (@p == null) {
      @p = create_prop();
      p.layer(15);
      p.sub_layer(1);
      p.prop_set(1);
      p.prop_group(1);
      p.prop_index(1);
      p.palette(1);
      g.add_prop(@p);
    }
    p.x(frame);

    ++frame;
  }

  void checkpoint_save() {
    frame_cp = frame;
  }

  void checkpoint_load() {
    frame = frame_cp;
    if (@p != null) {
      @p = prop_by_id(p.id());
    }
  }
}