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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 serialize_output(output):
 """ Serializes a transaction output. """

    if not ('value' in output and 'script_hex' in output):
        raise Exception('Invalid output')

    return ''.join([
        hexlify(struct.pack('<Q', output['value'])), # pack into 8 bites
        hexlify(variable_length_int(len(output['script_hex'])/2)),
        output['script_hex']
    ])