@Override
    public String toString() {
        String res = "		QuestionEquation";
        res = res + "\n			Text: " + this.text;
        res = res + "\n			Difficulty: " + this.difficulty;
        res = res + "\n			Operands: " + this.operands;
        res = res + "\n			Operators: " + this.operators;
        res = res + "\n			Unknowns: " + this.unknowns;
        res = res + "\n			Equation: ";
        Iterator<Integer> it_operands = this.operands.iterator();
        Iterator<Character> it_operators = this.operators.iterator();
        Iterator<Integer> it_unknowns = this.unknowns.iterator();
        res = res + it_operands.next();
        while (it_operands.hasNext()) {
            int u = it_unknowns.next();
            if (u == 1) {
                res = res + "x ";
            } else if (u == 2) {
                res = res + "x² ";
            } else {
                res = res + " ";
            }
            res = res + it_operators.next() + " ";
            res = res + it_operands.next();
        }
        int u = it_unknowns.next();
        if (u == 1) {
            res = res + "x ";
        } else if (u == 2) {
            res = res + "x² ";
        } else {
            res = res + " ";
        }
        return res;
    }