public void shiftStack () {
		if (stk_btm == -1) {//initialization
			stk_btm = 0;
		}
		if (stk[4].isEmpty()) {//if stk4 is empty, shift stk_pos to stk4
			while (!stk[stk_btm].isEmpty()) {
				stk[4].push(stk[stk_btm].pop_int());
			}
			stk[stk_btm].setEmpty();
			if (stk_btm == 3) {
				stk_btm = 0;
			}
			else stk_btm ++;
		}
	}