Board* Board::search(void) {
    for (int x=0; x<9; x++)
        for (int y=0; y<9; y++){
            if(!fs[x][y].assigned()){
                for (int d=1; d<=9; d++){
                    if(fs[x][y].in(d)){
			Board* copy = new Board(*this);
			bool test = (*copy).assign(x,y,d);
			if(test){
				return copy->search();
			}
                    }
                }
                return NULL;

            }
            else{
                return copy;
            }
        }
}