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