Union
$$A \cup B =$$
{ concrete operational, fable, fairy-tale, folk-tale, formal operational, myth, pre-operational, saga, sensorimotor }
Symmetric Difference
$$A \ominus B =$$
{ concrete operational, fable, fairy-tale, folk-tale, formal operational, myth, pre-operational, saga, sensorimotor }
Difference
$$A - B =$$
{ concrete operational, formal operational, pre-operational, sensorimotor }
Difference
$$B - A =$$
{ fable, fairy-tale, folk-tale, myth, saga }
Cartesian Product
$$A \times B =$$
{ (concrete operational, fable),
(concrete operational, fairy-tale),
(concrete operational, folk-tale),
(concrete operational, myth),
(concrete operational, saga),
(formal operational, fable),
(formal operational, fairy-tale),
(formal operational, folk-tale),
(formal operational, myth),
(formal operational, saga),
(pre-operational, fable),
(pre-operational, fairy-tale),
(pre-operational, folk-tale),
(pre-operational, myth),
(pre-operational, saga),
(sensorimotor, fable),
(sensorimotor, fairy-tale),
(sensorimotor, folk-tale),
(sensorimotor, myth),
(sensorimotor, saga) }
Cognition