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

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

    class Record(klass, ):
        value = pvector_field(int, optional=True)
    assert ((Record(value=[1, 2]).value, Record(value=None).value) == (pvector([1, 2]), None))
