$\forall$$l_{1}$, $l_{2}$:IdLnk, $d_{1}$, $d_{2}$:${\it tg}$:Id fp$\rightarrow$ Type.
\\[0ex]($l_{1}$ $=$ $l_{2}$ $\Rightarrow$ $d_{1}$ $\parallel$ $d_{2}$) $\Rightarrow$ lnk{-}decl($l_{1}$;$d_{1}$) $\parallel$ lnk{-}decl($l_{2}$;$d_{2}$)