Union
$$A \cup B =$$
{ context-free, context-sensitive, feeling, intuition, recursively enumerable, regular, sensation, thinking }
Symmetric Difference
$$A \ominus B =$$
{ context-free, context-sensitive, feeling, intuition, recursively enumerable, regular, sensation, thinking }
Difference
$$A - B =$$
{ context-free, context-sensitive, recursively enumerable, regular }
Difference
$$B - A =$$
{ feeling, intuition, sensation, thinking }
Cartesian Product
$$A \times B =$$
{ (context-free, feeling),
(context-free, intuition),
(context-free, sensation),
(context-free, thinking),
(context-sensitive, feeling),
(context-sensitive, intuition),
(context-sensitive, sensation),
(context-sensitive, thinking),
(recursively enumerable, feeling),
(recursively enumerable, intuition),
(recursively enumerable, sensation),
(recursively enumerable, thinking),
(regular, feeling),
(regular, intuition),
(regular, sensation),
(regular, thinking) }
Chomsky hierarchy