\begin{tabbing}
$\forall$$A$, $B$:Top.
\\[0ex]Rinterface($A$ $\oplus$ $B$)
\\[0ex]$\sim$
\\[0ex]let ${\it rec}_{1}$\= = Rinterface($A$) in\+
\\[0ex]let ${\it rec}_{2}$\= = Rinterface($B$) in\+
\\[0ex]if Rnone?(${\it rec}_{1}$) then ${\it rec}_{2}$ if Rnone?(${\it rec}_{2}$) then ${\it rec}_{1}$ else ${\it rec}_{1}$ $\oplus$ ${\it rec}_{2}$ fi 
\-\-
\end{tabbing}