“…The rules {∨ 1 , ∨ 2 , ∧, a , [a] , µ, ν} are called logical rules. The rules ∨ 1 , ∨ 2 , ∧, µ, ν are standard and also the rules a , [a] for reasoning about modalities are natural counterparts to the analogous rules adopted in proof systems for modal (fixed point) logics appeared in the literature (see, e.g., [19], [21] and [15]). The rules {δ, + λ , { }} are called distribution rules and constitute a crucial aspect of the system.…”