expand_prefix
expand_prefix(bt, lp->parent, lbound, &lprefix);
expand_prefix(bt, up->parent, ubound, &uprefix);
expand_prefix(bt, low, 0, &srckey);
expand_prefix(bt, src, 0, &tmpkey);
expand_prefix(bt, dst, 0, &tmpkey);
expand_prefix(bt, low, 0, &tmpkey);
expand_prefix(bt, src, i, &tmpkey);
static void expand_prefix(struct btree *bt, struct mpage *mp,