$\forall$$T$:Type, $R$:($T$$\rightarrow$$T$$\rightarrow\mathbb{P}$). Preorder($T$;$x$,$y$.$R$($x$,$y$)) $\Rightarrow$ EquivRel($T$;$a$,$b$.Symmetrize($x$,$y$.$R$($x$,$y$);$a$;$b$))