void AddPowerOfTen(BigInt & A, int N)
// postcondition: A contains the value of A + 10^N    
{
    BigInt tenpower = 1;
    int k;
    for(k=0; k < N; k++)
    {
	tenpower *= 10;
    }

    // tenpower now holds the value 10^N

    A += tenpower;
}