HPREV
retval = HPREV(h, ev);
retval = HPREV(h, &ev), i++) {
for (retval = HCURR(h, ev); retval != -1; retval = HPREV(h, ev))
for (retval = HCURR(h, ev); retval != -1; retval = HPREV(h, ev))
for (retval = HCURR(h, ev); retval != -1; retval = HPREV(h, ev))