/* factorial
   -- precondition: n >= 0
   -- postcondition: returns n! */
int factorial(int n)
{
  int k, product;
  if ( n < 0 ) return 0;
  product = 1;
  for ( k = 2; k <= n; k++ )
    /* loop invariant: product = (k-1)! */
    product = k*product;
  return product;
}