$\forall$$n$:$\mathbb{N}$, $T$:Type, $R_{1}$, $R_{2}$:($T$$\rightarrow$$T$$\rightarrow\mathbb{P}$). $R_{1}$ =$>$ $R_{2}$ $\Rightarrow$ rel\_exp($T$;$R_{1}$;$n$) =$>$ rel\_exp($T$;$R_{2}$;$n$)