InvFuns($A$;$B$;$f$;$g$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$($g$ o $f$) = Id\{$A$\} $\in$ $A$$\rightarrow$$A$ \& ($f$ o $g$) = Id\{$B$\} $\in$ $B$$\rightarrow$$B$