My text states that $f:\mathbb{N_n}\rightarrow \mathbb{N_n}$ where $f(i) = n-i$ is a bijection.
I am not convinced about this because if $i=n$, then $f(n)=n-n=0$ but $0 \notin \mathbb{N_n}$. Therefore, $f$ is not a function.
Is the text implicitly assuming $0 \in \mathbb{N_n}$ in this particular example? Previous examples of $\mathbb{N_n}$ states that $\mathbb{N_n}=\{1,2,\dots, n\}$.