private Operator nextOperator() throws IOException {
		int ttype = tokenizer.nextToken();
		
		if (ttype == StreamTokenizer.TT_EOF ||
				ttype == StreamTokenizer.TT_EOL) {
			return Operator.END;
		}
		if (ttype >= 0) {
			char sign = (char) tokenizer.ttype;
			if (sign == ')') {
				return Operator.END;
			}
			return Operator.parseOperator(sign);
		}
		throw new IllegalFormulaException("数式の終わり方が不正です");
	}