code: func (f *fragment) forEachBit(fn func(rowID, columnID uint64) error) error {
  	f.mu.Lock()
  	defer f.mu.Unlock()
  
  	var err error
  	f.storage.ForEach(func(i uint64) {
  		// Skip if an error has already occurred.
  		if err != nil {
  			return
  		}
  
  		// Invoke caller's function.
  		err = fn(i/ShardWidth, (f.shard*ShardWidth)+(i%ShardWidth))
  	})
  	return err
  }
docs: // forEachBit executes fn for every bit set in the fragment.
  // Errors returned from fn are passed through.