def get_agency_hierarchy_design_session(self, proxy=None):
if proxy is None:<EOL><INDENT>raise NullArgument()<EOL><DEDENT>raise Unimplemented()<EOL>
Gets the session designing agency hierarchies.

        arg:    proxy (osid.proxy.Proxy): a proxy
        return: (osid.authentication.AgencyHierarchyDesignSession) - an
                ``AgencyHierarchyDesignSession``
        raise:  NullArgument - ``proxy`` is null
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_agency_hierarchy_design()``
                is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_agency_hierarchy_design()`` is ``true``.*