bt_shrink_leaf
bt_shrink_leaf(tree, leaf, idx, 1);
bt_shrink_leaf(tree, neighbor, l_hdr->bth_count - 1, 1);
bt_shrink_leaf(tree, neighbor, 0, 1);
bt_shrink_leaf(tree, leaf, idx, 1);
bt_shrink_leaf(tree, leaf, keep_count, 1 + move_count);
bt_shrink_leaf(tree, l_neighbor, l_hdr->bth_count - move_count,