code: func (b *Blob) PartsSize() int64 {
  	n := int64(0)
  	for _, part := range b.ss.Parts {
  		n += int64(part.Size)
  	}
  	return n
  }
docs: // PartsSize returns the number of bytes represented by the "parts" field.
  // TODO: move this off *Blob to a specialized type.