double power(double x,int n)
{
  if (n == 0)
    return 1.0;

  if(x == 0)
    {
      printf("Invalid Base: '0' ");
      return 1;
    }
  if(n < 0)
    {
      x = 1.0/x;
      n = -n;
    }
  demand(n>0, power failed);
  return power2(x, n);
}