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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 _pyxb_from_perm_dict(self, perm_dict):
 """Return an AccessPolicy PyXB representation of ``perm_dict`` - If ``norm_perm_list`` is empty, None is returned. The schema does not allow AccessPolicy to be empty, but in SystemMetadata, it can be left out altogether. So returning None instead of an empty AccessPolicy allows the result to be inserted directly into a SystemMetadata PyXB object. """

        norm_perm_list = self._norm_perm_list_from_perm_dict(perm_dict)
        return self._pyxb_from_norm_perm_list(norm_perm_list)