$\forall$$l$:IdLnk, $k$:Knd, ${\it dt}_{1}$, ${\it dt}_{2}$:${\it tg}$:Id fp$\rightarrow$ Type.
\\[0ex]$k$ $\in$ dom(lnk{-}decl($l$;${\it dt}_{1}$ $\oplus$ ${\it dt}_{2}$)) $\sim$ ($k$ $\in$ dom(lnk{-}decl($l$;${\it dt}_{1}$)) $\vee_{b}$$k$ $\in$ dom(lnk{-}decl($l$;${\it dt}_{2}$)))