$\forall$$X$, $Y$:${\it ik}$:LocKnd fp$\rightarrow$ Top, ${\it ik}$:LocKnd.
\\[0ex]${\it ik}$ $\in$ dom(interface{-}union($X$;$Y$)) $\sim$ (${\it ik}$ $\in$ dom($X$) $\vee_{b}$${\it ik}$ $\in$ dom($Y$))