docs: private long allocate_from_tail(int block_size)
      throws IOException
      {
          long block_to_alloc;
  
          block_to_alloc = tail_ptr;
          tail_ptr = tail_ptr + block_size;
  
          allocated_blocks++;
          allocated_words += block_size;
  
          seek_and_count(block_to_alloc);
          writeInt(- block_size);
          seek_and_count(TAIL_DPTR);
          writeLong(tail_ptr);
  
          return(block_to_alloc + HDR_SIZE);
      }