1. $A$ : Type
\\[0ex]2. $B$ : Type
\\[0ex]3. $\exists$$f$:$A$$\rightarrow$$B$. Bij($A$;$B$;$f$)
\\[0ex]$\vdash$  $\exists$$f$:$A$$\rightarrow$$B$. $\exists$$g$:$B$$\rightarrow$$A$. InvFuns($A$;$B$;$f$;$g$)