public void addEntity(Entity entity) {
		CogaenId id = entity.getId();
		Entity old = this.entitiesMap.put(id, entity);
		if (old != null) {
			this.entitiesMap.put(id, old);
			throw new RuntimeException("ambiguous entity id " + id);
		}
		this.entities.add(entity);
		entity.engage();
	}