BigNumber operator += (const BigNumber&);

BigNumber BigNumber::operator += (const BigNumber& B)
{

    int a=0;
    for(int i=79;i>=1;i--)
    {
        number[i]=(number[i]+B.number[i]+a)%10;
        a=(number[i]+B.number[i]+a)/10;
    }
    number[0]=a+number[0]+B.number[0];
    if(number[0]>9)
    {
        throw "Error";
    }
    else
    {
        return (*this);
    }
}