get_nth_bh
bh = get_nth_bh(bh, bufnum);
bh = get_nth_bh(bh, bufnum);
bh = get_nth_bh(bh, block - first_block);
bh_frozen = get_nth_bh(bh_frozen,
bh_frozen = get_nth_bh(bh_frozen, n);
bh = get_nth_bh(bh, block - first_block);