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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 get_extra(descriptor):
 """ Python-specific helper to access "extra" field of descriptors, because it's not as straight-forward as in C. Returns a list, where each entry is an individual extra descriptor. """

    result = []
    extra_length = descriptor.extra_length
    if extra_length:
        extra = buffer_at(descriptor.extra.value, extra_length)
        append = result.append
        while extra:
            length = _string_item_to_int(extra[0])
            if not 0 < length <= len(extra):
                raise ValueError(
                    'Extra descriptor %i is incomplete/invalid' % (
                        len(result),
                    ),
                )
            append(extra[:length])
            extra = extra[length:]
    return result