'If ``optional`` argument is true, ``None`` is acceptable alternative
to a set.'

@given(PYRSISTENT_STRUCT)
def test_optional(self, klass):

    class Record(klass, ):
        value = pmap_field(int, int, optional=True)
    self.assertEqual((Record(value={1: 2}).value, Record(value=None).value), (pmap({1: 2}), None))
