// The size of the content that produced the second checksum must be provided.
// Not all checksum types can be merged, use the CanMerge method to check.
// Checksum types must match.
func (c *Checksum) AddPart(other Checksum, size int64) error {
	if !other.Type.CanMerge() {
		return fmt.Errorf("checksum type cannot be merged")
	}
	if size == 0 {
		return nil
	}
	if !c.Type.Is(other.Type.Base()) {