docs: private void add_block_to_freelist(Block block, int ql_index)
      {
          block.next = ql_heads[ql_index].first_block;
          ql_heads[ql_index].first_block = block;
          ql_heads[ql_index].length++;
          if (ql_heads[ql_index].length == 1) {
              nonempty_lists++;
          }
      }