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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def from_bytes(b):
 """ Generates either a HDPrivateKey or HDPublicKey from the underlying bytes. The serialization must conform to the description in: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#serialization-format Args: b (bytes):
 A byte stream conforming to the above. Returns: HDPrivateKey or HDPublicKey: Either an HD private or public key object, depending on what was serialized. """

        if len(b) < 78:
            raise ValueError("b must be at least 78 bytes long.")

        version = int.from_bytes(b[:4], 'big')
        depth = b[4]
        parent_fingerprint = b[5:9]
        index = int.from_bytes(b[9:13], 'big')
        chain_code = b[13:45]
        key_bytes = b[45:78]

        rv = None
        if version == HDPrivateKey.MAINNET_VERSION or version == HDPrivateKey.TESTNET_VERSION:
            if key_bytes[0] != 0:
                raise ValueError("First byte of private key must be 0x00!")

            private_key = int.from_bytes(key_bytes[1:], 'big')
            rv = HDPrivateKey(key=private_key,
                              chain_code=chain_code,
                              index=index,
                              depth=depth,
                              parent_fingerprint=parent_fingerprint)
        elif version == HDPublicKey.MAINNET_VERSION or version == HDPublicKey.TESTNET_VERSION:
            if key_bytes[0] != 0x02 and key_bytes[0] != 0x03:
                raise ValueError("First byte of public key must be 0x02 or 0x03!")

            public_key = PublicKey.from_bytes(key_bytes)
            rv = HDPublicKey(x=public_key.point.x,
                             y=public_key.point.y,
                             chain_code=chain_code,
                             index=index,
                             depth=depth,
                             parent_fingerprint=parent_fingerprint)
        else:
            raise ValueError("incorrect encoding.")

        return rv