$\forall$$A$:Type, ${\it eq}$:EqDecider($A$), $f$, $g$:$a$:$A$ fp$\rightarrow$ Top, $x$:$A$.
\\[0ex]$x$ $\in$ dom($f$ $\oplus$ $g$) $\sim$ ($x$ $\in$ dom($f$) $\vee_{2}$ $x$ $\in$ dom($g$))