question: public static Hash merge ( Hash a , Hash b ) { try { MessageDigest digest = MessageDigest . getInstance ( "SHA-256" ) ; digest . update ( a . bytes ) ; return Hash . createFromSafeArray ( digest . digest ( digest . digest ( b . bytes ) ) ) ; } catch ( NoSuchAlgorithmException e ) { throw new RuntimeException ( e ) ; } }
target:  Merge two Hashes into one for Merkle Tree calculation