Set the qualifier for the element.

        Verifies the element is allowed to have a qualifier, and
        throws an exception if not.

def set_qualifier(self, value):
        """Set the qualifier for the element.

        Verifies the element is allowed to have a qualifier, and
        throws an exception if not.
        """
        if self.allows_qualifier:
            self.qualifier = value.strip()
        else:
            raise UNTLStructureException(
                'Element "%s" does not allow a qualifier' % (self.tag,)
            )