$\forall$$A$:Type, ${\it eq}$:EqDecider($A$), $f$,$g$:fpf($A$; $a$.top), $x$:$A$.
\\[0ex]sqequal(fpf{-}dom(${\it eq}$; $x$; fpf{-}join(${\it eq}$; $f$; $g$)); bor(fpf{-}dom(${\it eq}$; $x$; $f$); fpf{-}dom(${\it eq}$; $x$; $g$)))