/* returns x with the n bits that begin at position p set to the
 * rightmost n bits of y, leaving the other bits unchanged.
 */

int setbits(int x, int p, int n, int y) {

     return (x & ~(~(~0 << n) << (p+1-n))) | 
	  (y & (~(~0 << n) << (p+1-n)));
}