$\forall$$A$:Type, $B$:($A$$\rightarrow$Type), ${\it eq}$:EqDecider($A$), $f$, $g$:$a$:$A$ fp$\rightarrow$ $B$($a$), $x$:$A$.
\\[0ex]$x$ $\in$ dom($f$ $\oplus$ $g$) $\Leftrightarrow$ $x$ $\in$ dom($f$) $\vee$ $x$ $\in$ dom($g$)