the preposition will moving when its sail is decreased to one
square_sail is not sail. if steer does not occur then travel occurs. fore_and_aft_sail is not sail. sail is not structure. head_sail is not sail. press_of_sail is not sail. if bear_down_on occurs then sail occurs. if bear_down_on does not occur then sail occurs. sailboats_only_move_if_wind is not blowing.