func Sum(bz []byte) []byte {
	hash := sha256.Sum256(bz)
	return hash[:Size]
}
func Sum(bz []byte) []byte {
	hash := sha256.Sum256(bz)
	return hash[:Size]
}
func Sum ( bz [ ] byte ) [ ] byte { hash := sha256 . Sum256 ( bz ) return hash [ : Size ] }
// Sum returns the first 20 bytes of SHA256 of the bz.
Sum returns the first 20 bytes of SHA256 of the bz .