$\forall$$T$:Type, $f$:($T$$\rightarrow$$T$). retraction($T$;$f$) $\Rightarrow$ ($\forall$$x$, $y$:$T$. $y$ is $f$$\ast$($x$) $\Rightarrow$ $x$ is $f$$\ast$($y$) $\Rightarrow$ ($x$ = $y$))