docs: public void coalesce()
      throws IOException
      {
          if (readOnly) {
              throw (new FileManagerException("Attempt to coalesce in read only mode"));
          }
          free_blocks = 0;
          free_words = 0;
          allocated_blocks = 0;
          allocated_words = 0;
          init_freelists(false);
          read_block_sizes_from_disk(true);
          coalesces++;
      }