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