abstract class OperatorExpression : IExpression {
    IExpression A;
    IExpression B;

    internal void Init!(IExpression a, IExpression b) {
        A = a;
        B = b;
    }

    internal abstract double OperatorImplementation(double a, double b);

    internal override double Calculate() => OperatorImplementation(A.Calculate(), B.Calculate());
}

class OperatorExpressionAdd : OperatorExpression {
    internal override double OperatorImplementation(double a, double b) => a+b;
}

class OperatorExpressionSub : OperatorExpression {
    internal override double OperatorImplementation(double a, double b) => a-b;
}

class OperatorExpressionMul : OperatorExpression {
    internal override double OperatorImplementation(double a, double b) => a*b;
}

class OperatorExpressionDiv : OperatorExpression {
    internal override double OperatorImplementation(double a, double b) => a/b;
}

class OperatorExpressionMod : OperatorExpression {
    // returns if b>0 then returns a%b else returns a
    internal override double OperatorImplementation(double a, double b) {
        if(b <= 0)
            return a;
        if(a > 0){
            while(a >= b)
                a -= b;
        }
        else {
            while(a <= b)
                a += b;
        }
        return a;
    }
}

class OperatorExpressionPow : OperatorExpression {
    internal override double OperatorImplementation(double a, double b) => Math.Pow(a, b);
}