Some remarks on Došen's logic $\mathsf{N}$ and its extensions

This paper collects some observations about Došen's logic $\mathsf{N}$, where negation is treated as a modal operator, and its extensions. We shall see what happens when we add the contraposition axiom to several important extensions of $\mathsf{N}$, show that certain extensions of $\mathsf{N}$ are canonical, and also revisit the method of filtration.