/* Midterm practice #6.  Implement the following function according to
 * the specification.  Follow the C-assembly interface conventions of
 * the lab's compiler and assembler.
 *
 * In:  l - lower bound
 *      u - upper bound
 *      s - step amount
 *      rv - pointer to where to write result
 * Out: -1 - if overflow occurred
 *       0 - if no error
 * Computes
 *   l + (l + s) + (l + 2s) + ... + (l + ks), where l + (k+1)s > u
 * For example, asum(-3, 4, 2, &x) computes
 *   -3 + -1 + 1 + 3
 * and writes the sum to the location of x.
 */
short asum(long l, long u, long s, long * rv);