$\forall$$A$,$B$:Type.
\\[0ex]($\forall$$x$,$y$:$A$. decidable(($x$ = $y$)))
\\[0ex]$\Rightarrow$ ($\forall$$u$,$v$:$B$. decidable(($u$ = $v$)))
\\[0ex]$\Rightarrow$ ($\forall$$x$,$y$:($A$ + $B$). decidable(($x$ = $y$)))