Soak the beans in water overnight to soften them. Next morning, change the water and boil them for about 15 minutes. Change the water again and add the onions, celery and finely chopped green peppers, chopped tomatoes, salt, pepper and oil. Allow all to boil for 20 minutes and then add the carrots cut into rounds. Remove from the heat after about ½ hour, when the beans have cooked and the sauce has thickened.