code: func (dig2f digestToFilterMapping) String() string {
  	var buffer bytes.Buffer
  	collection2TxID := make(map[string][]string)
  	for dig := range dig2f {
  		collection2TxID[dig.Collection] = append(collection2TxID[dig.Collection], dig.TxId)
  	}
  	for col, txIDs := range collection2TxID {
  		buffer.WriteString(fmt.Sprintf("{%s: %v}", col, txIDs))
  	}
  	return buffer.String()
  }
docs: // String returns a string representation of t he digestToFilterMapping