Errata and Addenda to 'Finite non-deterministic semantics for some modal systems'.

In this note, an error in the axiomatization of Ivlev’s modal system which we inadvertedly reproduced in our paper “Finite non-deterministic semantics for some modal systems” (CFP2015), is fixed. Additionally, some axioms proposed in (CFP2015) were slightly modified. All the technical results in (CFP2015) which depend on the previous axiomatization were also fixed. Finally, the discussion about decidability of the level valuation semantics initiated in (CFP2015) is taken up. The error in Ivlev’s axiomatization was originally pointed out by H. Omori and D. Skurt in the paper “More modal semantics without possible worlds”, where an alternative solution was proposed.