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