q{-}rel{-}lub($r_{1}$;$r_{2}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$if ($r_{1}$ =$_{0}$ 0) then $r_{2}$ if ($r_{2}$ =$_{0}$ 0) then $r_{1}$ if ($r_{1}$ =$_{0}$ 1) $\wedge_{b}$ ($r_{2}$ =$_{0}$ 1) then 1 else 2 fi