'If ``optional`` argument is ``False`` then ``pmap_field`` is
non-optional, i.e. does not allow ``None``.'

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

    class Record(klass, ):
        value = pmap_field(int, int, optional=False)
    self.assertRaises(AttributeError, Record, value=None)
