$\forall$$T$:Type, ${\it eq}$:EqDecider($T$), $d_{1}$, $d_{2}$:$a$:$T$ fp$\rightarrow$ Type.
\\[0ex]non{-}void($d_{1}$) $\Rightarrow$ non{-}void($d_{2}$) $\Rightarrow$ non{-}void($d_{1}$ $\oplus$ $d_{2}$)