'``pmap_field`` results in a map that enforces its value type.'

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

    class Record(klass, ):
        value = pmap_field(int, type(None))
    record = Record(value={1: None})
    self.assertRaises(TypeError, record.value.set, 2, 4)
