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