Regular modal logic

In modal logic, a regular modal logic L is a modal logic closed under the duality of the modal operators:

◊ A ≡ ¬ ◻ ¬ A

and the rule

( A ∧ B ) → C ⊢ ( ◻ A ∧ ◻ B ) → ◻ C .

Every regular modal logic is classical, and every normal modal logic is regular and hence classical.