HIGH_MASK
"HIGH MASK ", hashp->HIGH_MASK,
hashp->HIGH_MASK = (nbuckets << 1) - 1;
if (new_bucket > hashp->HIGH_MASK) {
hashp->LOW_MASK = hashp->HIGH_MASK;
hashp->HIGH_MASK = new_bucket | hashp->LOW_MASK;
bucket = n & hashp->HIGH_MASK;